Elementary operators and their applications

Operator Theory: Advances and Applications Volume 212 Founded in 1979 by Israel Gohberg Editors: Joseph A. Ball (Black...

Author: Curto P.E. | Mathieu M. (eds.)

176 downloads 1215 Views 2MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Operator Theory: Advances and Applications Volume 212 Founded in 1979 by Israel Gohberg

Editors: Joseph A. Ball (Blacksburg, VA, USA) Harry Dym (Rehovot, Israel) Marinus A. Kaashoek (Amsterdam, The Netherlands) Heinz Langer (Vienna, Austria) Christiane Tretter (Bern, Switzerland) Associate Editors: Vadim Adamyan (Odessa, Ukraine) Albrecht Böttcher (Chemnitz, Germany) B. Malcolm Brown (Cardiff, UK) Raul Curto (Iowa, IA, USA) Fritz Gesztesy (Columbia, MO, USA) Pavel Kurasov (Lund, Sweden) Leonid E. Lerer (Haifa, Israel) Vern Paulsen (Houston, TX, USA) Mihai Putinar (Santa Barbara, CA, USA) Leiba Rodman (Williamsburg, VA, USA) Ilya M. Spitkovsky (Williamsburg, VA, USA)

Subseries Linear Operators and Linear Systems Subseries editors: Daniel Alpay (Beer Sheva, Israel) Birgit Jacob (Wuppertal, Germany) André C.M. Ran (Amsterdam, The Netherlands) Subseries Advances in Partial Differential Equations Subseries editors: Bert-Wolfgang Schulze (Potsdam, Germany) Michael Demuth (Clausthal, Germany) Jerome A. Goldstein (Memphis, TN, USA) Nobuyuki Tose (Yokohama, Japan) Ingo Witt (Göttingen, Germany)

Honorary and Advisory Editorial Board: Lewis A. Coburn (Buffalo, NY, USA) Ciprian Foias (College Station, TX, USA) J.William Helton (San Diego, CA, USA) Thomas Kailath (Stanford, CA, USA) Peter Lancaster (Calgary, Canada) Peter D. Lax (New York, NY, USA) Donald Sarason (Berkeley, CA, USA) Bernd Silbermann (Chemnitz, Germany) Harold Widom (Santa Cruz, CA, USA)

Elementary Operators and Their Applications 3rd International Workshop held at Queen’s University Belfast, 14–17 April 2009

Raúl E. Curto Martin Mathieu Editors

Editors Raúl E. Curto Department of Mathematics University of Iowa 14 MacLean Hall Iowa City, IA 52242-1419 USA [email protected]

Martin Mathieu Department of Pure Mathematics Queen’s University Belfast University Road Belfast BT7 1NN Northern Ireland [email protected]

2010 Mathematics Subject Classification: Primary 47B47; Secondary 46-06, 46A32, 46H35, 46H99, 46L05, 46L07, 47-06, 47A05, 47A10, 47A13, 47A62, 47B05

ISBN 978-3-0348-0036-5 e-ISBN 978-3-0348-0037-2 DOI 10.1007/978-3-0348-0037-2 Library of Congress Control Number: 2011920820 © Springer Basel AG 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. For any kind of use permission of the copyright owner must be obtained. Cover design: deblik, Berlin Printed on acid-free paper Springer Basel AG is part of Springer Science+Business Media www.birkhauser-science.com

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

Picture of Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

viii

List of Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix

Programme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

x

Abstracts of Talks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

Contributions N. Boudi and M. Mathieu Elementary Operators that are Spectrally Bounded . . . . . . . . . . . . . . . . .

1

D. Kitson The Browder Spectrum of an Elementary Operator . . . . . . . . . . . . . . . . .

17

B. Magajna Approximation of Maps on C ∗ -Algebras by Completely Contractive Elementary Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

P. Rosenthal Some Not-quite-elementary Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41

V.S. Shulman and Yu.V. Turovskii Topological Radicals, II. Applications to Spectral Theory of Multiplication Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

V.S. Shulman and L. Turowska An Elementary Approach to Elementary Operators on B(H) . . . . . . . . 115 R.M. Timoney Computation Versus Formulae for Norms of Elementary Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

133

Open Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

151

Introduction

The Third International Workshop on Elementary Operators and Their Applications was held at the Department of Pure Mathematics of Queen’s University Belfast between April 14 and 17, 2009. It was organised by Dr Martin Mathieu as a satellite to the joint meeting of the British Mathematical Colloquium and the Annual Meeting of the Irish Mathematical Society that took place at NUI Galway the week before. The funding received from the Irish Mathematical Society and the London Mathematical Society is gratefully acknowledged. This series of workshops started in June 1991 at the Heinrich Fabri-Institute of the University of T¨ ubingen, Germany and was continued in September 2001 at the University of Helsinki, Finland. At each of these meetings substantial progress in the research on elementary operators was reported. The proceedings volume of the 1991 workshop listed a number of difficult problems some of which, notably the norm problem for elementary operators have been solved in the meantime. The present volume once again aims to present the state-of-the-art of our understanding of the theory of elementary operators and their applications. In fact, the applications are far more wide-ranging than one might expect and touch on many areas in Mathematics but also in Physics, such as Solid State Physics and Quantum Information Theory. Elementary operators are so simple in their definition that they occur everywhere; for instance, every linear mapping on a finite-dimensional semisimple Banach algebra is an elementary operator. It is no surprise therefore that one tries to approximate various classes of operators in infinite dimensions by elementary operators; some of the contributions in this book discuss these. Yet the simple definition of an elementary operator entirely conceals the intricate and often challenging interplay between structural properties of elementary operators and the underlying algebras. This volume contains solicited articles by speakers at the workshop ranging from expository surveys to original research papers, each of which carefully refereed. They all bear witness to the very rich mathematics that is connected with the study of elementary operators, may it be multivariable spectral theory, the invariant subspace problem or tensor products of C*-algebras.

viii

Introduction

As always in mathematics, results lead to new questions, and therefore the volume concludes with another list of open problems some of which were explained in the workshop problem session. May they inspire further exciting insights on elementary operators!

31 July 2010

Ra´ ul E Curto, Iowa City Martin Mathieu, Belfast

A picture of the workshop participants taken by the organiser

List of Participants Jer´ onimo Alaminos, Universidad de Granada, Granada, Spain Rob Archbold, University of Aberdeen, Aberdeen, Scotland Ariel Blanco, Queen’s University Belfast, Belfast, Northern Ireland Nadia Boudi, Université Moulay Ismail, Meknès, Morocco Ra´ ul Curto, University of Iowa, Iowa City, USA Lawrence Fialkow, SUNY at New Paltz, New Paltz, USA Robin Harte, Trinity College Dublin, Dublin, Ireland Derek Kitson, Trinity College Dublin, Dublin, Ireland Rupert Levene, Trinity College Dublin, Dublin, Ireland Bojan Magajna, University of Ljubljana, Ljubljana, Slovenia Martin Mathieu, Queen’s University Belfast, Belfast, Northern Ireland Martin McGarvey, Queen’s University Belfast, Belfast, Northern Ireland F. Javier Meri, Granada, Universidad de Granada, Granada, Spain Savvas Papapanagides, Queen’s University Belfast, Belfast, Northern Ireland Robert Pluta, Trinity College Dublin, Dublin, Ireland Peter Rosenthal, University of Toronto, Toronto, Canada Eero Saksman, University of Helsinki, Helsinki, Finland Victor Shulman, Vologda State Technical University, Vologda, Russia Franek Szafraniec, Jagiellonian University, Krak´ ow, Poland Richard Timoney, Trinity College Dublin, Dublin, Ireland Ivan Todorov, Queen’s University Belfast, Belfast, Northern Ireland Ta Ngoc Tri, Lancaster University, Lancaster, England Aleksej Turnˇsek, University of Ljubljana, Ljubljana, Slovenia Lyudmila Turowska, Chalmers University of Technology and the University of Gothenburg, Gothenburg, Sweden Hans-Olav Tylli, University of Helsinki, Helsinki, Finland Armando R. Villena, Universidad de Granada, Granada, Spain

Ariel Blanco

Peter Rosenthal

16:00 ʹ 16:50

17:00 ʹ 17:50

19:30

Reception

Coffee Break

15:30 ʹ 16:00

18:00 ʹ 19:00

Ivan Todorov

Victor Shulman

15:00 ʹ 15:30

14:00 ʹ 14:50

Welcome

Richard Timoney

11:30 ʹ 12:20

Football Match YƵĞĞŶ͛ƐW Or Black Taxi Tour through Belfast

Coffee Break

Franek Szafraniec

Raúl Curto

Lunch Break

Coffee Break

11:00 ʹ 11:30

Registration

Robin Harte

10:30 ʹ 11:00

12:30 ʹ 14:00

Rob Archbold

Wednesday, 15 April

9:30 ʹ 10:20

Tuesday, 14 April

Conference Dinner

Eero Saksman

Aleksej Turnsek

Coffee Break

Derek Kitson

Larry Fialkow

Lunch Break

Nadia Boudi

Coffee Break

Martin McGarvey

Bojan Magajna

Thursday, 16 April

End of workshop

Lunch

Hans-Olav Tylli

Coffee Break

Problem Session

Lyudmila Turowska

Friday, 17 April

3rd International Workshop on Elementary Operators and their Applications (ElOp2009) Programme

x Programme

Abstracts of Talks

States, representations and norms of elementary operators Rob Archbold University of Aberdeen [email protected] Coauthors: Douglas Somerset (Aberdeen) and Richard Timoney (Dublin) We discuss recent work with Douglas Somerset and Richard Timoney which aims to localize the connection between matricial norms of elementary operators on a C ∗ -algebra and the weak∗ -approximation of factorial states. Let T be an elementary operator on a C ∗ -algebra A, π an irreducible representation of A and T π the induced operator on π(A). It is automatic that if n is a positive integer then T π n ≤ T n and that the n-positivity of T implies the n-positivity of T π . We show that if the upper multiplicity MU (π) > 1 then T π k ≤ T n for certain values of k > n and that the n-positivity of T implies the k-positivity of T π for these same larger values of k. The results are obtained by using Timoney’s descriptions of matricial norms and k-positivity in terms of the “tracial geometric mean” and factorial states. The condition on MU (π) allows one to approximate factorial states associated with π by type I factorial states of lower degree. These localizations at irreducible representations π lead to new proofs of various characterizations of the class of antiliminal-by-abelian C ∗ -algebras in terms of factorial states and elementary operators. On the cohomology of Banach operator algebras Ariel Blanco Queen’s University Belfast [email protected] We present new results on the bounded cohomology of Banach operator algebras. Some of them will be generalizations of well-known results for properly infinite C ∗ -algebras.

xii

Abstracts of Talks

On spectrally bounded elementary operators Nadia Boudi Université Moulay Ismail nadia [email protected] I will first recall basic results on spectrally bounded linear maps, and then I will describe some spectrally bounded elementary operators. At the end of my talk, I will discuss some related conditions. 2-variable weighted shifts built from elementary tensors Ra´ ul E. Curto University of Iowa [email protected] Coauthors: Sang Hoon Lee and Jasang Yoon We consider the class T C of 2-variable weighted shifts with tensor core. These are shifts whose restrictions to a large invariant subspace split as (I ⊗ Wα , Wβ ⊗ I), so these restrictions are canonically associated to left and right multiplication operators. For the class T C, we study the Reconstruction-of-the-Measure Problem (ROMP), which consists of finding necessary and sufficient conditions on a pair T ≡ (T1 , T2 ) ∈ T C that guarantee its subnormality. ROMP is intimately connected to the Lifting Problem for Commuting Subnormals (LPCS), which asks for necessary and sufficient conditions for a pair of subnormal operators on Hilbert space to admit commuting normal extensions. It is well known that the commutativity of the pair is necessary but not sufficient, and it has recently been shown that the joint hyponormality of the pair is necessary but not sufficient. Moreover, while abstract solutions of LPCS exist, concrete solutions are only known in very specific situations. Our previous research has shown that many of the nontrivial aspects of LPCS are best detected within the class H1 of commuting hyponormal pairs of subnormal operators, so we focus our attention on this class. T C is a large subclass of H1 , and we have been able to provide a complete solution of LPCS within T C. We also show that for a pair T ∈ T C, the subnormality of (T12 , T2 ) (typically easier to verify) automatically implies the subnormality of T. Abstract vs. concrete solutions to multivariable truncated moment problems Lawrence Fialkow SUNY New Paltz [email protected] In previous work with R.E. Curto, we formulated two solutions to the truncated multivariable moment problem, one in terms of “flat extensions” of positive moment matrices, another in terms of positive extensions of linear functionals on

Abstracts of Talks

xiii

polynomial spaces associated with the moment data. These solutions are difficult to implement concretely. We discuss some recent concrete results concerning multivariable quadratic moment problems and the bivariatic quartic moment problem (joint work with Jiawang Nie), and a concrete solution to the y = x3 truncated moment problem. Hermitian subspaces and Fuglede operators (Contributed talk) Robin Harte Trinity College Dublin [email protected] A real-linear subspace H ⊆ A of a complex linear algebra with identity 1 will be called a “Hermitian subspace” if it satisfies H ∩ iH = {0} ; 1 ∈ H . There is induced an involution ∗ : h + ik → h − ik on the complex linear subspace H + iH ⊆ A. When A = B(X) is the bounded linear operators on a Banach space an involution on H + iH gives rise to “Fuglede” operators T ∈ H + iH for which T −1 (0) ⊆ T ∗−1 (0) . There is also induced another involution on an appropriate subspace of the “elementary operators” on A = B(X). In this discussion we relate the Fuglede property for T and S † to Fuglede properties for LT − RS and LT RS . A multivariable spectral mapping theorem (Contributed talk) Derek Kitson Trinity College Dublin [email protected] The classical notions of ascent and descent for a linear operator on a vector space can be extended to arbitrary collections of operators. Using this fact we construct a Browder joint spectrum for commuting n-tuples of bounded operators on a complex Banach space. This Browder joint spectrum is compact valued and satisfies a multivariable spectral mapping theorem. Connections to the Browder spectrum of an elementary operator will be discussed. Approximation by elementary operators Bojan Magajna University of Ljubljana [email protected] On which C ∗ -algebras A can all complete contractions, that preserve closed two-sided ideals, be approximated by completely contractive elementary operators? In the operator norm topology this is possible only if A is a finite direct sum of homogeneous C ∗ -algebras arising from C ∗ -bundles of finite type. On the other hand, pointwise approximation is always possible if A is nuclear, but the precise characterization is still open.

xiv

Abstracts of Talks

Normalisers, nest algebras and tensor products (Contributed talk) Martin McGarvey Queens University Belfast [email protected] If A is an operator algebra acting on a Hilbert space H, a normaliser of A is an operator T on H such that T ∗ AT ⊆ A and T AT ∗ ⊆ A. The set of all normalisers of A is denoted by N (A). We will show that if A is the tensor product of finitely many continuous nest algebras, B is a CDCSL algebra and N (A) = N (B) then either A = B or A = B∗ . Some not-quite-elementary operators Peter Rosenthal University of Toronto [email protected] Coauthors: Don Hadwin and Eric Nordgren This will be a discussion of joint work with Don Hadwin and Eric Nordgren concerning operator equations such as AXB + CY D = Z. Alexandrov measures and connectedness in the space of composition operators Eero Saksman University of Helsinki [email protected] Coauthors: Eva Gallardo (Zaragoza), Maria Gonzales (Cadiz) and Pekka Nieminen (Helsinki) We show that compact composition operators on the Hardy space H 2 do not form a component in the norm topology of operators. This answers a question of J.H. Shapiro and C. Sundberg from the early 1990’s. On elementary operators with compact coefficients Victor Shulman Vologda State Technical University shulman [email protected] Coauthors: Yuri Turovskii W. Wojtynsky proved in 1970 that each solution of a linear operator equation A XB of the form k k + AX + XB = λX, where Ak , Bk , A, B are compact k operators on a Banach space and λ = 0, is a nuclear operator. We suggest a new approach to this result and extend it considering eigenvectors and spectral subspaces of elementary operators with sufficiently many compact coefficients and of operators of more general form (in particular infinite sums are admitted).

Abstracts of Talks

xv

On Murphy’s positive definite kernels and Hilbert C ∗ -modules (Contributed talk) Franciszek Hugon Szafraniec Jagiellonian University Krakow [email protected] The paper the title refers to is that in Proceedings of the Edinburgh Mathematical Society, 40 (1997), 367–374. Taking it as an excuse we intend to realize a twofold purpose: 1. to atomize that important result showing by the way connections which are out of favour; 2. to rectify a tiny piece of history. The objective 1 is going to be achieved by adopting means adequate to goals; it is of great gravity and this is just Mathematics. The other, 2, comes from the author’s internal need of showing how ethical values in Mathematics are getting depreciated. The latter have nothing to do with the previous issue; the coincidence is totally accidental. Computation versus formulae for norms of elementary operators Richard Timoney Trinity College Dublin [email protected] We survey long standing and newer results on norms of elementary operators, including those of Stampfli (on derivations) and Haagerup (on completely bounded norms). We consider them from the point of view of their effectiveness for a practical problem and their value as a theoretical device. We discuss some aspects where there may be scope for further progress. s-numbers of elementary operators (Contributed talk) Ivan Todorov Queen’s University Belfast [email protected] Coauthors: M. Anoussis and V. Felouzis A theorem of Fong and Sourour states that an elementary operator acting on B(H) is compact if and only if it has a representation where its symbols are compact operators. In this talk, a quantitative version of this result will be presented where the behaviour of the s-numbers of an elementary operator is linked to the behaviour of the singular numbers of its symbols. Orthogonality and Fuglede–Putnam theorem Aleksej Turnsek University of Ljubljana [email protected] We present some results on orthogonality of the range and the kernel of elementary operators and connect them with the Fuglede–Putnam theorem.

xvi

Abstracts of Talks

Beurling–Pollard type theorems Lyudmila Turowska Chalmers University Gothenburg [email protected] Coauthors: Victor Shulman We establish a version of Beurling–Pollard theorem for operator synthesis and apply it to derive some results on linear operator equations. In addition, we establish a Beurling–Pollard theorem for weighted Fourier algebra and use it to obtain ascent estimate for operators that are functions of generalized scalar operators. This is a joint work with Victor Shulman. Two-sided multiplication operators on L(Lp ) Hans-Olav Tylli University of Helsinki [email protected] I will discuss the following results from [LST] and [JS] about the qualitative properties of the basic multiplication operators LA RB , where S → ASB for fixed operators A, B ∈ L(Lp ), on the space L(Lp ) of the bounded operators on Lp . (i) LA RB is strictly singular L(Lp ) → L(Lp ) for 1 ≤ p ≤ ∞ if and only if A = 0 and B = 0 are strictly singular operators on Lp, [LST], (ii) LA RB is weakly compact L(Lp ) → L(Lp ) for 2 < p < ∞ if and only if either A is compact, B is compact, or JA ∈ G2 and B ∈ Gp for some isometry J : Lp → L∞ , [JS]. Here A ∈ L(Lp ) belongs to the factorization ideal Gr if A = U V , where U ∈ L(r , Lp ) and V ∈ L(Lp , r ).

References W.B. Johnson and G. Schechtman, Multiplication operators on L(Lp ) and p strictly singular operators, J. Eur. Math. Soc. 10 (2008), 1105–1119. [LST] M. Lindstr¨ om, E. Saksman and H.-O. Tylli, Strictly singular and cosingular multiplications, Canad. J. Math. 57 (2005), 1249–1278. [JS]

Elementary Operators that are Spectrally Bounded Nadia Boudi and Martin Mathieu Abstract. We study spectrally bounded elementary operators of length two on a complex unital Banach algebra A. Some related conditions are investigated as well. In particular, we show that if S is an elementary operator of length two such that S(x) is quasi-nilpotent for every x ∈ A, then S(x)3 ∈ radA for every x ∈ A. Mathematics Subject Classification (2000). 47B47; 46H99, 47B48. Keywords. Banach algebras, elementary operators, spectrally bounded, finite spectrum.

1. Introduction Let A be a unital complex Banach algebra with Jacobson radical radA. For x ∈ A, let r(x) denote the spectral radius of x. A linear mapping T : A → A is said to be spectrally bounded if there is M > 0 such that r(T x) ≤ M r(x) for every x ∈ A [14]. A systematic study of such operators was begun in [16], and since then many results, in particular on the structure of spectrally bounded operators and spectral isometries (i.e., spectral radius preserving mappings) have been obtained, see [9, 10, 11, 17, 18] to cite but a few. The paper [15] discusses some of the main open problems in this area. As usual, La and Ra stand for the left and the right multiplication by a ∈ A, respectively. Using the subharmonicity of the spectral radius Pt´ ak proved in [20] that La is spectrally bounded if and only if a commutes with every element in A modulo radA. (For an alternative argument, see [12].) Curto and Mathieu showed that the generalised inner derivation La − Rb , a, b ∈ A is spectrally bounded if and This paper developed out of a collaboration started during the 3rd International Conference on Elementary Operators and their Applications held at Queen’s University Belfast in April 2009. The first-named author thanks the second-named author for the generous hospitality during this meeting. The support by the London Mathematical Society is gratefully acknowledged.

R.E. Curto and M. Mathieu (eds.), Elementary Operators and Their Applications, Operator Theory: Advances and Applications 212, DOI 10.1007/978-3-0348-0037-2_1, © Springer Basel AG 2011

1

2

N. Boudi and M. Mathieu

only if both La and Rb are spectrally bounded [12, Theorem B], thus extending Breˇsar’s earlier result for inner derivations La − Ra from [7]. Little seems to be n known in this direction about general elementary operators S = j=1 Laj Rbj despite the fact that, for instance, the question when all S ∈ E(A), the algebra of elementary operators on A, are spectrally infinitesimal (that is, Sx is quasinilpotent for all x ∈ A) is related to other properties of A; see [19, Section 6] for example. In this paper we make a start on studying elementary operators that are spectrally bounded by focussing on the case of length two. More precisely, we investigate the following three properties a linear mapping T : A → A can have: (a) T is spectrally bounded; (b) T is almost spectrally bounded; (c) T x has finite spectrum for all x ∈ A. It turns out that both (a) and (c) each imply (b) but clearly (a) ⇒ (c) (just take the identity mapping) and (c) ⇒ (a): left multiplication La with a a non-central element in a finite-dimensional semisimple Banach algebra A serves as an example. Operators with the property that every element in their range has finite spectrum have recently attracted some attention; see [8, 6, 5], for example. They appear naturally in our context, see Example 3.3 below. The finite spectrum condition often implies that the range (of the operator itself or a related one) is contained in the socle socA of A whereas spectrally boundedness is related to having the range lying in the radical radA. It therefore comes as no surprise that some of our results are along those lines too, see Corollary 3.6 and Theorem 3.9. An elementary operator leaves each ideal of A invariant; hence it is close at hand to use primitive ideals and dense algebras of operators to study properties of it. The Jacobson Density Theorem, and its variants, prevail as a major tool in our discussion too.

2. Length two elementary operators Let a, b ∈ A and denote by Ma,b the two-sided multiplication x → axb, x ∈ A. Since r(axb) = r(bax), it follows immediately from the aforementioned characterisation by Pt´ ak that Ma,b is spectrally bounded if and only if ba ∈ Z(A), the centre modulo the radical of A, which is defined by Z(A) = {z ∈ A | zx − xz ∈ radA for all x ∈ A}. A length two elementary operator is of the form S = Ma,b + Mc,d , where the sets {a, c} and {b, d} are linearly independent. (In fact, a stronger form of independence may have to be imposed in order to make it impossible to write such S as a length one operator Mu,v depending on the properties of the centre of A. For C*-algebras, this is studied in detail in [2, Chapter 5], however for general Banach algebras the situation is far more complicated. Therefore, strictly speaking we consider elementary operators of length at most two but this slight ambiguity does not affect our results below.)

Elementary Operators that are Spectrally Bounded

3

Without further assumptions, the two-sided multiplication Ma,b is not a homomorphism; however, if ba is a non-zero scalar, φ = (ba)−1 Ma,b is easily checked to be multiplicative. This observation is the key to a sufficient criterion for spectrally boundedness of length two elementary operators in this section. Lemma 2.1. Let A be a unital Banach algebra, and let φ, ψ be two bounded homomorphisms on A such that ψ(x)φ(y) ∈ radA for every x, y ∈ A. Then, for all λ, μ ∈ C, the mapping T = λφ + μψ is spectrally bounded. Proof. Fix x ∈ A. Take a non-zero complex number α ∈ σ(λx) ∪ σ(μx). Then there exists y ∈ A such that λ λ x+y−y x=0 α α and hence λ λ φ(x) + φ(y) − φ(y) φ(x) = 0. α α Analogously, there exists z ∈ A such that μ μ ψ(x) + ψ(z) − ψ(z) ψ(x) = 0. α α We compute that μ λ (2.1) 1 − φ(y) − ψ(z) 1 − φ(x) − ψ(x) = 1 + s, α α λ where s = αμ φ(y)ψ(x) + α ψ(z)φ(x). It follows from our assumption that s2 ∈ radA and hence s is quasi-nilpotent. This implies that 1 + s is invertible which, together with identity (2.1), entails that α − (λφ + μψ)(x) is left invertible. Since the boundary of the spectrum of T x is contained in the left approximate point spectrum, it follows that α ∈ / ∂σ(T x) and thus

∂σ(T x) ⊆ σ(λx) ∪ σ(μx). This implies that r(T x) ≤ max{|λ|, |μ|} r(x) for each x ∈ A which completes the proof. Lemma 2.2. Let A be a unital Banach algebra. Let ψ be a bounded homomorphism on A and let φ be a linear mapping on A such that φ(x)2 ∈ radA for every x ∈ A. Suppose that ψ(x)φ(y) ∈ radA for all x, y ∈ A or φ(x)ψ(y) ∈ radA for all x, y ∈ A. Then the mapping T = φ + ψ is spectrally bounded. Proof. Let x ∈ A and let α be a non-zero complex number such that α ∈ σ(x). There exists y0 ∈ A such that ψ(x) ψ(x) + ψ(y0 ) − ψ(y0 ) = 0. (2.2) α α Suppose first that ψ(x)φ(y) ∈ radA for all x, y ∈ A. From identity (2.2) we obtain φ(x) ψ(x) φ(x) φ(x) − =1− + ψ(y0 ) . (2.3) 1 − ψ(y0 ) 1 − α α α α

4


As in the proof of Lemma 2.1 we deduce that α − T x is left invertible and hence r(T x) ≤ r(x) for each x. Suppose now that φ(x)ψ(y) ∈ radA for all x, y ∈ A. Identity (2.3) together 2 − ψ(y0 ) φ(x) ∈ radA allow us to complete the proof as with the fact that φ(x) α α before. Throughout the following, PrimA shall denote the set of primitive ideals of A. An elementary operator S ∈ E(A) leaves each ideal of A invariant. Therefore, for each P ∈ PrimA, it induces an elementary operator SP ∈ E(AP ) on the primitive Banach algebra AP = A/P . The well-known formula, [3, Theorem 4.2.1], σ(xP ) (x ∈ A), (2.4) σ(x) = P ∈PrimA

where xP = x + P , enables us to reduce the situation to the primitive case: If each SP is spectrally bounded and the spectral operator norms SP σ (in the sense of [16]) are uniformly bounded, then S is spectrally bounded as r(Sx) = sup r(SP xP ) ≤ sup SP σ r(xP ) ≤ sup SP σ r(x) P

P

(x ∈ A).

P

Proposition 2.3. Let A be a unital Banach algebra, and let S = Ma,b + Mc,d ∈ E(A). Suppose that ba, dc ∈ Z(A) and bc ∈ radA. Then S is spectrally bounded. Proof. Let P be a primitive ideal of A. By hypothesis, bP cP = 0 and bP aP = λP , dP cP = μP are both scalars. If λP = μP = 0 then we compute that (SP xP )3 = 0 for each x ∈ A and thus SP σ = 0. If λP = 0 and μP = 0 we put φP = λ−1 P MaP ,bP and ψP = μ−1 M to obtain two (bounded and spectrally bounded) homomorc ,d P P P phisms. Since ψP (xP )φP (yP ) = 0 for all x, y ∈ A, we can apply Lemma 2.1 to conclude that SP = λP φP + μP ψP is spectrally bounded with SP σ ≤ max{r(ba), r(dc)}. If, say, λP = 0 and μP = 0 then the hypotheses of Lemma 2.2 are fulfilled, for φP = MaP ,bP and ψP = μ−1 P McP ,dP , and we obtain SP σ ≤ 1. The discussion preceding this lemma show that S is spectrally bounded with Sσ ≤ max{r(ba), r(dc), 1}.

3. Main results Clearly the condition on the coefficients in Proposition 2.3 is not necessary for S to be spectrally bounded; simply take b = −c = 1 and refer back to the results cited in the Introduction. We shall obtain a necessary and sufficient condition in Theorem 3.5 below. We shall also explore a weaker condition, n-almost spectrally boundedness in this section and obtain a necessary condition as well. Our techniques are inspired by [12], in particular the use of Jacobson’s Density Theorem and Sinclair’s improvement on it, see [3, Theorem 4.2.5] and [3, Corollary 4.2.6], respectively. We shall also require the following variant of Amitsur’s Lemma [1], which is obtained in [5, Lemma 2.1].


5

Lemma 3.1. Let n, r be positive integers, U and V vector spaces over C and let V0 be a finite-dimensional subspace of V . Let T1 , . . . , Tn : U → V be linear mappings. Suppose that, for every r-tuple of vectors ζ1 , . . . , ζr in U , the set {T1 ζi , . . . , Tn ζi : i = 1, . . . , r} is linearly dependent modulo V0 . Then there exists a non-trivial linear combination of T1 , . . . , Tn of finite rank. Let X be a non-empty set. As usual, the cardinality of X will be denoted by #X. Throughout, A is a unital Banach algebra and n ≥ 1 a natural number. Definition 3.2. Let T : A → A be a linear mapping. We say that T is n-almost spectrally bounded if there exists M > 0 such that, for every x ∈ A, # λ ∈ σ(T x) : |λ| > M r(x) ≤ n. (3.1) In this case, M is called an n-almost spectral bound for T . Clearly, every spectrally bounded linear mapping is n-almost spectrally bounded for each n. Here is a less trivial example. Example 3.3. Let T : A → A be a bounded linear mapping satisfying #σ(T x) < ∞ for each x ∈ A. It follows from [8, Lemma 2.1] that there exists n ∈ N such that #σ(T x) ≤ n for all x ∈ A. Thus T is n-almost spectrally bounded. For a Banach space X, let B(X) denote the algebra of all bounded linear operators on X, and F (X) denote the ideal of all finite rank operators in B(X). Set F (X) = F (X) + CI, where I is the identity mapping. Proposition 3.4. Suppose that S = Ma,b + Mc,d ∈ E(A) is n-almost spectrally bounded for some n ∈ N. Then, for every irreducible representation π of A on a Banach space X, there exists β ∈ C such that π((b + βd)a) ∈ F (X)

and

π(d(c − βa)) ∈ F (X)

and either π(b + βd)π(c − βa) has finite rank or β = 0 and π(da) has finite rank. Proof. Let π be an irreducible representation of A on a Banach space X. In the case when X is finite dimensional, there is nothing to prove; so we shall assume that X is infinite dimensional. Suppose first that there exist r ∈ N and ζ1 , . . . , ζr ∈ X such that the set {ζj , π(ba)ζj , π(da)ζj : 1 ≤ j ≤ r} is linearly independent. By [3, Corollary 4.2.6], for every k ∈ N, we can find xk ∈ exp A such that, for each j, span {ζj , π(ba)ζj , π(da)ζj } is invariant under π(xk ) and the corresponding matrix representation of π(xk ) with respect to {ζj , π(ba)ζj , π(da)ζj } is ⎛ ⎞ 0 j 0 ⎝ 1 0 0⎠ . k 0 0 1 Choose x ∈ A with the property that π(x)ζj = π(ba)ζj and π(xda)ζj = 0, for 1 ≤ j ≤ r. Set xk = xk xx−1 k . Then π S(xk )a ζj = jk π(a)ζj

6


so that {k, . . . , rk} ⊆ σ(πS(xk )) ⊆ σ(S(xk )). Let M be an n-almost spectral bound for S. If r > n, then, for each k ∈ N, we must have k ≤ M r(x) which is impossible. It follows that r ≤ n and thus the set {ζj , π(ba)ζj , π(da)ζj : 1 ≤ j ≤ n + 1} is linearly dependent for all vectors ζ1 , . . . , ζn+1 ∈ X. By Lemma 3.1, there exists a non-trivial linear combination of I, π(ba) and π(da) which has finite rank. By symmetry, some non-trivial linear combination of I, π(dc) and π(bc) has finite rank. Suppose first that π(ba) ∈ F (X) and π(dc) ∈ F (X) which allows us to write π(da + αba) = λI + F and π(bc + βdc) = λ I + F , where λ, λ , α, β ∈ C and F, F ∈ F (X). Assume towards a contradiction that, for each r ∈ N, there exist ζ1 , . . . , ζr ∈ X such that the set {ζj , π(ba)ζj , π(dc)ζj : 1 ≤ j ≤ r} is linearly independent modulo F X + F X. For k ∈ N, we can find xk ∈ exp A such that, for each j, span {ζj , π(ba)ζj , π(dc)ζj } is invariant under π(xk ), the corresponding matrix representation of π(xk ) with respect to {ζj , π(ba)ζj , π(dc)ζj } is ⎛ ⎞ 0 j 0 ⎝ 1 0 0⎠ k 0 0 1 and π(xk )|F X+F X = I. Choose x ∈ A with the property that π(x)ζj = π(ba)ζj , π(xba)ζj = π(xdc)ζj = 0 and π(x)F = π(x)F = 0, for 1 ≤ j ≤ r. Set xk = xk xx−1 k . Then π S(xk )a ζj = jk π(a − αc)ζj and π S(xk )c ζj = 0 so that {k, . . . , rk} ⊆ σ(πS(xk )). As above, we deduce that the set {ζj , π(ba)ζj , π(dc)ζj : 1 ≤ j ≤ n + 1} is linearly dependent modulo F X + F X for all vectors ζ1 , . . . , ζn+1 ∈ X. Applying once again Lemma 3.1, we infer that there exists a non-trivial linear combination of I, π(ba) and π(dc), which has finite rank. Write π(dc + δba) = γI + G, for some δ, γ ∈ C and G ∈ F (X). Next choose vectors ζ1 , . . . , ζr ∈ X such that the set {ζj , π(ba)ζj : 1 ≤ j ≤ r} is linearly independent modulo F X + F X + GX which is possible since π(ba) ∈ F (X). For every k ∈ N, we can find xk ∈ exp A such that, for each j, span {ζj , π(ba)ζj } is invariant under π(xk ), the corresponding matrix representation of π(xk ) with respect to {ζj , π(ba)ζj } is 0 1 1 0 jk


7

and π(xk )|F X+F X+GX = I. Choose x ∈ A with the property that π(x)ζj = π(ba)ζj , π(xba)ζj = π(ba)ζj , and π(x)F = π(x)F = π(x)G = 0. Set xk = xk xx−1 k . Then the corresponding matrix representation of πS(xk ) with respect to {π(a)ζj , π(c)ζj } is jk λ − βγ + βδjk . λ − αjk γ − δjk The trace of this matrix is (1 − δ)jk + γ. If δ = 1 we can choose k sufficiently big to obtain a contradiction as above. Thus δ = 1. The characteristic polynomial of the above matrix is χ(t) = t2 − γt − λ(λ − βγ) + (jk)2 (αβ − 1) + jk(αλ + γ − λβ − αβγ) and has to be independent of k; thus we must have αβ = 1 and λ = λβ 2 . Consequently, π(dc) + π(ba) ∈ F (X) and βπ(da) + π(ba) = βλI + βF

and απ(bc) + π(dc) = βλI + αF

from which it follows that π(b + βd)π(c − βa) ∈ F (X). Suppose now that π(ba) ∈ F (X) or π(dc) ∈ F (X). Observe that, for every α ∈ C, we have S = Ma+αc,b + Mc,d−αb = Ma,b−αd + Mc+αa,d .

(3.2)

By choosing a suitable α, we will reduce this situation to the previous one. We distinguish two cases: Case 1: π(ba) ∈ F (X) and π(da) ∈ / F (X), or π(dc) ∈ F (X) and π(bc) ∈ / F (X). Suppose, for instance, the first possibility. Then, for at most one complex numbers α, we have π(d(c + αa)) ∈ F (X) and π((b − αd)a) ∈ F (X). Using identity (3.2) and the above steps, we get the desired result. More precisely, we show that π(dc) ∈ F (X) and π(bc) ∈ F (X). Case 2: π(ba), π(da) ∈ F (X) or π(dc), π(bc) ∈ F (X). Assume, for instance, that π(ba), π(da) ∈ F (X). Suppose first that π(dc) ∈ F (X). We claim that either π(da) ∈ F (X) or π(bc) ∈ F (X). If π(da) ∈ F (X) we write π(da) = λI + F, π(dc) = γI + F , π(ba) = μI + G, where λ, γ, μ ∈ C and F, F , G ∈ F (X). Without loss of generality, we may suppose that λ = 1. Suppose that there are vectors ζ1 , . . . , ζr ∈ X such that {ζj , π(bc)ζj : 1 ≤ j ≤ r} is linearly independent modulo F X + F X + GX. For every k ∈ N, we can find xk ∈ exp A such that, for each j, span {ζj , π(bc)ζj } is invariant under π(xk ), the corresponding matrix representation of π(xk ) with respect to {ζj , π(bc)ζj } is 0 1 1 0 jk and π(xk )|F X+F X+GX = I. Choose x ∈ A with the property that π(x)ζj = π(bc)ζj , π(xbc)ζj = π(bc)ζj and π(x)F = π(x)F = π(x)G = 0 for all j. Set xk =

8


xk xx−1 k . Then the corresponding matrix representation of πS(xk ) with respect to {π(a)ζj , π(c)ζj } is μ jk . 1 γ

As above, we show that there exists r ∈ N such that the set {ζ1 , . . . , ζr , π(bc)ζ1 , . . . , π(bc)ζr } is linearly dependent for all vectors ζ1 , . . . , ζr in X. Thus, π(bc) ∈ F (X) and the claim is proved. Since π(bc), π(dc), π(ba) and π(da) now all belong to F (X), there exists a complex number β such that π(b + βd)π(c − βa) ∈ F (X), and the proof of this case is complete. Suppose finally that π(dc) ∈ / F (X). We already showed above that there exists β ∈ C such that π(bc + βdc) ∈ F (X). Re-writing S as in (3.2) with α = −β −1 provided β = 0 and α = 1 if β = 0, we see that we are in Case 1. Thus, this case cannot occur. Given P ∈ PrimA, we shall denote by πP an irreducible representation of A on a Banach space XP with kernel P . Theorem 3.5. Let A be a unital Banach algebra. Then S = Ma,b + Mc,d ∈ E(A) is spectrally bounded if and only if, for every primitive ideal P of A, there exists βP ∈ C such that πP ((b + βP d)a) ∈ CI

and

πP (d(c − βP a)) ∈ CI

(3.3)

and either πP (b + βP d)πP (c − βP a) = 0 or βP = 0 and πP (da) = 0. In particular, ba + dc ∈ Z(A) in this case. Proof. The “only if”-part is a particular case of the proof of Proposition 3.4. The space X can be finite dimensional, and at each step, we work with one ζ ∈ X instead of an r-tuple. To show the sufficiency of the condition, write πP ((b+βP d)a) = λP I for some λP ∈ C and note that (3.3) implies that uP ∈ CI for every P , where u = ba + dc. We shall show that the set {λP : P ∈ PrimA} ⊆ C is bounded. If |βP | < 12 then |λP | ≤ πP (ba) +

1 2

πP (da) ≤ ba + da.

Otherwise |βP | ≥ 12 , and the identity πP (b + βP d)πP (c − βP a) = 0 entails that βP πP ((b + βP d)a) = πP ((b + βP d)c) and thus |λP | ≤ 2 πP (bc) + πP (dc) ≤ 2 bc + dc. Consequently there is M1 > 0 such that |λP | ≤ M1 for all P ∈ PrimA.


9

We can use this to infer that r(πP (d(c − βP a))) = r(πP (ba + dc) − πP ((b + βP d)a)) = r(πP (u) − λP I) ≤ r(u) + M1 for all P ∈ PrimA. Therefore there is M2 > 0 such that r(πP (d(c − βP a))) ≤ M2 for all P . Writing SP once more as SP = MaP ,bP +βP dP + McP −βP aP ,dP = McP ,dP + MaP ,bP we apply Proposition 2.3 to conclude that each SP , P ∈ PrimA is spectrally bounded. Moreover, the proof of that proposition shows that SP σ ≤ max{M1 , M2 , 1}

(P ∈ PrimA)

from which we conclude that S itself is spectrally bounded.

We draw the following consequence for spectrally infinitesimal elementary operators of length two. Corollary 3.6. Let S = Ma,b + Mc,d ∈ E(A). If S(x) is quasi-nilpotent for every x ∈ A, then S(x)3 ∈ radA for every x ∈ A. Proof. Let P be a primitive ideal of A and let π = πP . According to Theorem 3.5 there exists β ∈ C such that π(ba + βda) ∈ CI

and π(ba + dc) ∈ CI,

and either π(b + βd)π(c − βa) = 0 or β = 0 and π(da) = 0. Writing S as S = Ma,b+βd + Mc−βa,d and replacing b by b + βd and c by c − βa, if necessary, we may assume that β = 0 and either π(bc) = 0 or π(da) = 0. Set π(ba) = λI and π(dc) = λ I. We shall prove that λ = λ = 0. Suppose that π(da) = 0 and π(a) = 0 (otherwise we clearly have λ = 0). Take x ∈ A and ζ ∈ X such that π(x)ζ = ζ

and π(a)ζ = 0.

Then π(S(x)a)ζ = λπ(a)ζ wherefore λ ∈ σ(S(x)) = {0}. It follows that π(S(x)a) = 0 for all x ∈ A. If λ = 0 we can find ζ ∈ X \ {0}, x ∈ A and μ ∈ C such that π(xbc)ζ = μζ and π(xdc)ζ = ζ. It follows that π(S(x)c)ζ = μπ(a)ζ + π(c)ζ. Upon applying π(S(x)) to this identity we arrive at a contradiction since π(S(x)a) = 0. The other possibility, i.e., π(bc) = 0, is treated analogously. In either case we conclude that, for every x ∈ A, πS(x)3 = 0. As a result, 3 S(x) ∈ radA for every x ∈ A. It follows from the above proof that ba + dc ∈ radA if S(x) is quasi-nilpotent for every x ∈ A. In the remainder of this article we focus our attention on elementary operators of length two such that every element in their range has finite spectrum. As observed above (Example 3.3) such an operator is n-almost spectrally bounded, where n is a uniform bound for the number of scalars in the spectrum.

10


Proposition 3.7. Let S = Ma,b + Mc,d ∈ E(A) and suppose that σ(S(x)) is finite for every x ∈ A. Then, for each primitive ideal P of A, there exists βP ∈ C such that πP ((b + βP d)a), πP (d(c − βP a)) ∈ F (XP ) and either πP (b + βP d)πP (c − βP a) has finite rank or βP = 0 and πP (da) has finite rank. Proof. Let P ∈ PrimA and π = πP , X = XP . By [8, Lemma 2.1], there exists n ∈ N such that #σ(S(x)) < n for every x ∈ A and it therefore follows from Proposition 3.4 that there exists β ∈ C such that π((b + βd)a), π(d(c − βa)) ∈ F (X) and either π(b + βd)π(c − βa) has finite rank or β = 0 and π(da) has finite rank. Write π(ba + βda) = λI + F, where F ∈ F (X). Suppose that λ = 0 for every λ associated to P . In the case that π(ba) ∈ F (X), choose vectors ζ1 , . . . , ζr ∈ X, r ∈ N such that {ζ1 , . . . , ζr , π(ba)ζ1 , . . . , π(ba)ζr } is linearly independent modulo F X and pick x ∈ A such that π(xba)ζj = jζj , π(x(λ − ba))ζj ) = 0 and π(x)F = 0. Then π(S(x))π(a)ζj = jπ(a)ζj which implies that {1, . . . , r} ⊆ σ(S(x)). By choosing r sufficiently large, we obtain a contradiction. Next assume that π(ba) ∈ F (X)\F (X). Choose vectors ζ1 , . . . , ζr ∈ X such that {π(ba)ζ1 , . . . , π(ba)ζr } is linearly independent. Observe that either π(da) ∈ F (X) or π(da) ∈ / F (X). Therefore we may suppose without loss of generality that there exists x ∈ A such that π(xba)ζj = jζj and π(xda)ζj = 0. Once again we get a contradiction. Now suppose that π(ba) ∈ F (X). Then we must have π(da), π(dc) in F (X). By considering (S(x)a)ζj , (S(x)c)ζj for suitable x ∈ A and ζ1 , . . . , ζr ∈ X, we show that π(dc) ∈ F (X) and either π(bc) ∈ F (X) or π(da) ∈ F (X). Thus there exists β0 ∈ C, associated to P , such that π(ba + β0 da) ∈ F (X). Write S = Ma,b+β0 d + Mc−β0 a,d . We already know that π(dc − β0 da) ∈ F (X) and π((b + β0 d)(c − β0 a)) has finite rank. Assume that π(dc − β0 da) ∈ F (X). Then, for each r, there are vectors ζ1 , . . . , ζr ∈ X such that the set {π(dc−β0 da)ζ1 , . . . , π(dc− β0 da)ζr } is linearly independent modulo π((b + β0 d)(c − β0 a))X. Choose x ∈ A such that π(x(dc − β0 da))ζj = jζj and π(x(b + β0 d)(c − β0 a)) = 0. By considering S(x)(c − β0 a) and r sufficiently large, we get a contradiction. This completes the proof. For a semiprime algebra A, we denote by socA its socle, see [4, Chapter IV]. As usual, khsocA denotes the intersection of all primitive ideals of A containing


11

socA, the kernel of the hull of socA. It is easy to show that rad A/socA = kh socA/socA. Theorem 3.8. Let A be a unital Banach algebra, and let S = Ma,b + Mc,d ∈ E(A). If S is n-almost spectrally bounded for some n ∈ N, then, for all but finitely many primitive ideals P of A, the induced mapping SP : A/P → A/P is spectrally bounded. Moreover, there exists s ∈ A such that s + radA ∈ soc A/radA and ba + dc − s ∈ Z(A). Proof. Suppose there exist distinct primitive ideals P1 , . . . , Pr such that, for every / CI. To simplify the notation we shall write Xj , πj instead of γ ∈ C, πPj ((b+γd)a) ∈ XPj , πPj whenever convenient in the following. Fix 1 ≤ j ≤ r. By Proposition 3.4, there exists βj ∈ C such that πj ((b + βj d)a) ∈ F (XP ). Let ζj ∈ Xj be such that {πj (ba + βj da)ζj , ζj } is linearly independent. Choose xj ∈ A and, for every k ∈ N, choose xjk ∈ A such that span {ζj , πj (ba)ζj , πj (da)ζj } is invariant under πj (xjk ), πj (exp(xjk )(ba + βj da))ζj = ζj ,

1 πj (ba + βj da)ζj , jk πj (xj )ζj = πj (ba + βj da)ζj .

πj (exp(xjk ))ζj =

πj (xj )(ba + βj da)ζj = 0, Then π exp(xjk )xj exp(−xjk ) ζj = 0. Observe that either πj (da) ∈ CI or there exists ζj ∈ Xj such that {ζj , πj (da)ζj , πj (ba + βj da)ζj } is linearly independent. In both cases, we can suppose without loss of generality that πj exp(xjk )xj exp(−xjk )da ζj = 0.

Applying the Extended Jacobson Density Theorem [13, p. 283], we can find x ∈ A and, for each k, xk ∈ A such that πj (xk )ζ = πj (xjk )ζ,

πj (x)ζ = πj (xj )ζ

for every ζ ∈ span {ζj , πj (ba)ζj , πj (da)ζj }. Set xk = exp(xk )x exp(−xk ); then πj (Sxk )a ζj = jkπj (a)ζj . Thus {k, . . . , rk} ⊆ σ(Sxk ). As in the proof of Proposition 3.4, we show that r must be sufficiently small. We have thereby shown that for all but finitely many primitive ideals P of A, there exists γP ∈ C such that πP ((b + γP d)a) ∈ CI. Analogously, we show that for all but finitely many primitive ideals P of A, there exists γP ∈ C such that πP (d(c − γP a)) ∈ CI. Let Γ be the set of primitive ideals P for which there exists βP ∈ C with the property that πP ((b + βP d)c) ∈ F (XP ). Using similar arguments, we show that for all but finitely many primitive ideals P ∈ Γ, there exists ρP ∈ C such that πP (bc + ρP dc) ∈ CI. Now let r > n and let P1 , . . . , Pr be distinct primitive ideals in Γ. Suppose that for each 1 ≤ j ≤ r, we can find γj , γj , ρj ∈ C and λj , λj , μj ∈ C such that πj ((b + γj d)a) = λj I, πj (d(c − γj a)) = λj I and πj ((b + ρj d)c) = μj I. In the case that πj (da) ∈ CI,

12


we have πj (ba), πj (dc) and πj (bc) belong to CI so that there exists βj ∈ C with πj ((b + βj d)(c − βj a)) = 0. Suppose now that, for each j, there exists ζj ∈ Xj such that {ζj , πj (da)ζj } is linearly independent. Fix 1 ≤ j ≤ r and α1 , . . . , αr ∈ C. For each k, choose xjk ∈ A such that span {ζj , πj (da)ζj } is invariant under πj (xjk ), 1 πj (da)ζj πj exp(xjk ) ζj = αj k

and πj exp(xjk )da ζj = ζj .

Choose xj ∈ A with the property that πj (xj )ζj = πj (da)ζj

and πj (xj da)ζj = πj (da)ζj . The corresponding matrix representation of πj S(exp(xjk )xj exp(−xjk )) with respect to {πj (a)ζj , πj (c)ζj } is μj − λj ρj − αj kγj ρj λj − γj αj k . Bj = αj k λj + αj kγj Applying again the Extended Jacobson Density Theorem and arguing as above, we can find xk ∈ A such that r(xk ) = r(xk ) and Bj is the corresponding matrix representation of πj S(xk ) with respect to {πj (a)ζj , πj (c)ζj }. The trace of Bj is λj + λj + αj k(γj − γj ). Suppose that k is sufficiently large. Using suitable scalars αj , we see that γj = γj for at least r − n primitive ideals from {P1 , . . . , Pr }. The characteristic polynomial of Bj is χj (t) = t2 − (λj + λj )t − (γj2 − γj ρj )(jk)2 − (λj γj − λj γj + μj − λj ρj )jk + λj λj . As above, we see that χj (t) has to be independent of k for at least r − n primitive ideals from {P1 , . . . , Pr }. Of course, χj (t) is independent of k if and only if γj = ρj , and μj = λj γj or γj = 0 and μj = λj ρj . Thus, for all but finitely many P ∈ Γ, there exists βP ∈ C such that πP ((b + βP d)a), πP (d(c − βP a)) belong to CI and πP (b + βP d)πP (c − βP a) = 0. Next let Γ be the set of primitive ideals for which πP (ba), πP (dc), πP (da) belong to CI. In the same manner we see that for all but finitely many P ∈ Γ \ Γ we have πP (da) = 0. In summary we proved that for all but finitely many primitive ideals P of A, there exists βP ∈ C such that πP ((b + βP d)a) ∈ CI

and πP (d(c − βP a)) ∈ CI

(3.4)

and either πP (b + βP d)πP (c − βP a) = 0 or βP = 0 and πP (da) = 0. By Proposition 2.3, the induced map SP is spectrally bounded for all but finitely many primitive ideals P of A. By (3.4), for all but finitely many P ∈ PrimA, πP (ba + dc), πP (x) = 0 for all x ∈ A. For the remaining primitive ideals Q, πQ (ba + dc) ∈ F (XQ ) by Proposition 3.4 and hence πQ (ba + dc), πQ (x) has finite spectrum for all x ∈ A. It follows from (2.4) that #σ([ba + dc, x]) < ∞ for all x ∈ A. Using [6,


13

Theorem 2.4], we find that there exists s ∈ A such that s + radA ∈ soc A/radA and ba + dc − s ∈ Z(A). This completes the proof. The following theorem is our second main result. Theorem 3.9. Let A be a semisimple unital Banach algebra, and let S = Ma,b + Mc,d ∈ E(A). If every element in the range of S has finite spectrum then S(x)3 ∈ socA for every x ∈ A. Proof. Let P ∈ PrimA. Using the notation of Proposition 3.7 we write SP in the form SP = MaP ,bP +βP dP + McP −βP aP ,dP . (3.5) By the statement of Proposition 3.7, we check at once that, for each P , SP (xP )3 ∈ soc AP for all x ∈ A. By the proof of Theorem 3.8, for all but finitely many primitive ideals P , bP aP + βP dP aP = 0, dP cP + bP aP = 0 and either (bP + βP dP )(cP − βP aP ) = 0 or βP = 0 and dP aP = 0. Invoking (3.5) we find that SP (xP )3 = 0 in this case. Applying, e.g., [8, Proposition 2.2], we infer that (Sx)3 ∈ socA for all x ∈ A, as desired. We note some consequences for length one elementary operators. Corollary 3.10. Let Ma,b ∈ E(A) for a semisimple unital Banach algebra A. Then every element in the range of Ma,b has finite spectrum if and only if ba ∈ socA. In this case Ma,b (x)2 ∈ socA for all x ∈ A. Proof. Since σ(axb) ∪ {0} = σ(bax) ∪ {0} for each x ∈ A, the “if”-part is evident. For the “only if”-part we specialise Proposition 3.7 and the proof of Theorem 3.8 to the case at hand to obtain that bP aP ∈ soc AP for every P ∈ PrimA and bP aP = 0 except for at most finitely many P ’s. The characterisation of the socle given in [8, Proposition 2.2] allows us to conclude that ba ∈ socA. There is an obvious extension of Corollary 3.10 to general Banach algebras. Corollary 3.11. Let Ma,b ∈ E(A) be n-almost spectrally bounded for some n ∈ N. Then there exists s ∈ A such that s + radA ∈ soc A/radA and ba − s ∈ Z(A). Proof. This is immediate from Theorem 3.8.

Suppose that A is semisimple and let T : A → A be a linear operator. We say that T is essentially spectrally bounded if there exists M > 0 such that r T x + I(A) ≤ M r x + I(A) (x ∈ A), where I(A) = khsocA is the ideal of inessential elements in A (see [15] for this notion in a more general context). Corollary 3.12. Let Ma,b ∈ E(A) be n-almost spectrally bounded for some n ∈ N, where A is semisimple. Then Ma,b is essentially spectrally bounded.

14


Proof. Denote by Aˆ = A/I(A) the quotient Banach algebra and by Maˆ,ˆb the ˆ If Ma,b is n-almost spectrally bounded, then induced elementary operator on A. ˆbˆ ˆ a = ba+I(A) ∈ Z(A) by Corollary 3.11. Consequently, M ˆ is spectrally bounded a ˆ ,b

which is the same as the assertion.

We do not know if the same result holds for elementary operators of length two. Concluding Remark 3.13. In view of the above results it is tempting to surmise that, whenever A is semisimple and S is a spectrally bounded elementary operator of length two, there exist a, b, c, d ∈ A such that S = Ma,b + Mc,d and both Ma,b and Mc,d are spectrally bounded with orthogonal ranges. However, the centre of A, and that of a suitable Banach algebra enlargement of A, may not be well behaved enough to allow for this.

References [1] S.A. Amitsur, Generalized polynomial identities and pivotal monomials, Trans. Amer. Math. Soc. 114 (1965), 210–226. [2] P. Ara and M. Mathieu, Local multipliers of C*-algebras, Springer Monographs in Mathematics, Springer-Verlag, London, 2003. [3] B. Aupetit, A primer on spectral theory, Springer-Verlag, New York, 1991. [4] F.F. Bonsall and J. Duncan, Complete normed algebras, Springer-Verlag, Berlin, New York, 1973. [5] N. Boudi, On the product of derivations in Banach algebras, Math. Proc. Royal Irish Academy 109 A (2009), 201–211. [6] N. Boudi and M. Mathieu, Commutators with finite spectrum, Illinois J. Math. 48 (2004), 687–699. [7] M. Breˇsar, Derivations decreasing the spectral radius, Arch. Math. 61 (1993), 160– 162. ˇ [8] M. Breˇsar and P. Semrl, Derivations mapping into the socle, Math. Proc. Cambridge Phil. Soc. 120 (1996), 339–346. [9] C. Costara and D. Repovˇs, Spectral isometries onto algebras having a separating family of finite-dimensional irreducible representations, J. Math. Anal. Appl. 365 (2010), 605–608. [10] J. Cui and J. Hou, The spectrally bounded linear maps on operator algebras, Studia Math. 150 (2002), 261–271. ˇ [11] A. Foˇsner and P. Semrl, Spectrally bounded linear maps on B(X), Canad. Math. Bull. 47 (2004), 369–372. [12] R. Curto and M. Mathieu, Spectrally bounded generalized inner derivations, Proc. Amer. Math. Soc. 123 (1995), 2431–2434. [13] J.M.G. Fell and R.S. Doran, Representations of *-algebras, locally compact groups, and Banach *-algebraic bundles, I, Academic Press, New York, 1988.


15

[14] M. Mathieu, Where to find the image of a derivation, Banach Center Publ. 30 (1994), 237–249. [15] M. Mathieu, A collection of problems on spectrally bounded operators, Asian-Eur. J. Math. 2 (2009), 487–501. [16] M. Mathieu and G.J. Schick, First results on spectrally bounded operators, Studia Math. 152 (2002), 187–199. [17] M. Mathieu and G.J. Schick, Spectrally bounded operators from von Neumann algebras, J. Operator Theory 49 (2003), 285–293. [18] M. Mathieu and A.R. Sourour, Hereditary properties of spectral isometries, Arch. Math. 82 (2004), 222–229. [19] A. Moreno Galindo and A. Rodr´ıguez Palacios, Topologically nilpotent normed algebras, preprint 2010. [20] V. Pt´ ak, Derivations, commutators and the radical, Manuscripta Math. 23 (1978), 355–362. Nadia Boudi Département de Mathématiques Université Moulay Ismail Faculté des Sciences Meknes, Maroc e-mail: nadia [email protected] Martin Mathieu Department of Pure Mathematics Queen’s University Belfast Belfast BT7 1NN Northern Ireland e-mail: [email protected]

The Browder Spectrum of an Elementary Operator Derek Kitson Abstract. We relate the ascent and descent of n-tuples of multiplication operators Ma,b (u) = aub to that of the coefficient Hilbert space operators a, b. For example, if a = (a1 , . . . , an ) and b∗ = (b∗1 , . . . , b∗m ) have finite non-zero ascent and descent s and t, respectively, then the (n + m)-tuple (La , Rb ) of left and right multiplication operators has finite ascent and descent s + t − 1. Using these results we obtain a description of the Browder joint spectrum of (La , Rb ) and provide formulae for the Browder spectrum of an elementary operator acting on B(H) or on a norm ideal of B(H). Mathematics Subject Classification (2000). Primary 47B49; Secondary 47A13. Keywords. Ascent, descent, joint spectrum.

Introduction The Browder spectrum of a Banach space operator is obtained by removing from the ordinary spectrum all eigenvalues of finite multiplicity which are poles of the resolvent. Let H be a complex Hilbert space and B(H) the collection of bounded operators on H. An elementary operator on B(H) is an operator of the form E : B(H) → B(H), u → a1 ub1 + · · · + an ubn where a = (a1 , . . . , an ), b = (b1 , . . . , bn ) ∈ B(H)n . Spectral properties of elementary operators have been considered by various authors (see [2, 3, 5, 6, 7, 9, 10]). In this article we obtain formulae for the Browder spectrum of E. These formulae may have applications to Weyl and Browder type theorems for elementary operators. The ascent of a linear mapping a acting on a vector space X is usually described as the length of the increasing chain of null spaces {0} ⊆ ker a ⊆ ker a2 ⊆ ker a3 ⊆ · · ·


17

18

D. Kitson

This value is also the smallest non-negative integer r for which ker a∩ran ar = {0}. The descent of a is the length of the decreasing chain of range spaces X ⊇ ran a ⊇ ran a2 ⊇ ran a3 ⊇ · · · . . . , an ) be and equals the smallest r for which ker ar + ran a = X. Let a = (a1 , an n-tuple of linear mappings on a vector space X. We write N (a) = nj=1 ker aj n and R(a) = j=1 ran aj . We denote by ar the lexicographically ordered nr -tuple of operators on X consisting of all products aj1 . . . ajr . For example if n = 2 then a = (a1 , a2 ) a2 = (a21 , a1 a2 , a2 a1 , a22 ) a3 = (a31 , a21 a2 , a1 a2 a1 , a1 a22 , a2 a21 , a2 a1 a2 , a22 a1 , a32 ) etc. In the terminology of [8], the ascent of a = (a1 , . . . , an ) on X is the smallest r such that N (a) ∩ R(ar ) = {0}. The descent of a = (a1 , . . . , an ) on X is the smallest r such that N (ar )+R(a) = X. If the ascent and descent of a = (a1 , . . . , an ) are both finite then they are equal ([8, Proposition 1.9]). We will denote the ascent of an n-tuple a = (a1 , . . . , an ) by α(a, X) or simply α(a) if the space X is understood. Similarly we write δ(a, X) or simply δ(a) for the descent of a = (a1 , . . . , an ). For operators a, b ∈ B(H) define the multiplication operator Ma,b : B(H) → B(H), u → aub. With I the identity operator on H, the left multiplication operator is La = Ma,I and the right multiplication operator is Rb = MI,b . Given an n-tuple a = (a1 , . . . , an ) of operators on H we write La = (La1 , . . . , Lan ) and Ra = (Ra1 , . . . , Ran ). More generally, we will consider multiplication operators acting on ideals I of B(H) such as the compact operators, the trace-class operators, the Hilbert-Schmidt operators and all p-Schatten classes of operators. In Section 1 we show that if a = (a1 , . . . , an ) and b∗ = (b∗1 , . . . , b∗m ) have finite non-zero ascent and descent s and t respectively then the (n + m)-tuple (La , Rb ) = (La1 , . . . , Lan , Rb1 , . . . , Rbm ) has finite ascent and descent s + t − 1. m We also obtain bounds for the ascent and descent of Ma,b = (Mai ,bj )n, i=1,j=1 . In [8] a Browder joint spectrum σb was introduced for n-tuples of Banach space operators. In Section 2 we obtain a description of the Browder joint spectrum of (La , Rb ) and collect formulae for the Browder spectrum of an elementary operator acting on B(H) or on a norm ideal of B(H), σb (E) = σl (a) ◦ σb− (b) ∪ σb+ (a) ◦ σr (b) ∪ σr (a) ◦ σb+ (b) ∪ σb− (a) ◦ σl (b) = (σH (a) ◦ σb (b)) ∪ (σb (a) ◦ σH (b)) = (σT (a) ◦ σT b (b)) ∪ (σT b (a) ◦ σT (b)) Here σl and σr are the left and right spectra, σb+ and σb− are analogues of the semi-Browder spectra of an operator, σH is the Harte spectrum, σT is the Taylor spectrum and σT b is the Taylor–Browder spectrum.

The Browder Spectrum of an Elementary Operator

19

1. Ascent and descent Theorem 1.1. Let a = (a1 , . . . , an ) be an n-tuple of operators on a Hilbert space H. Let I be a left ideal of B(H). Then (i) α(La , I) = α(a, H) (ii) δ(La , I) = δ(a, H) Proof. (i) If u ∈ N (La ) ∩ R((La )r ) then ran u ⊆ N (a) ∩ R(ar ). We conclude that α(La ) ≤ α(a). For the reverse inequality, suppose x ∈ N (a) ∩ R(ar ). Then x = ai1 . . . air (xi1 ...ir ) for some xi1 ...ir ∈ H. Choose y ∈ H such that x, y = 1. For each xi1 ...ir consider the rank one operator pi1 ...ir ∈ I given by pi1 ...ir (z) = z, yxi1 ...ir for all z ∈ H. Let p = ai1 . . . air pi1 ...ir . Then p ∈ N (La ) ∩ R((La )r ) and x = p(x). We conclude that α(a) ≤ α(La ). (ii) Suppose N ((La )r ) + R(La ) = I. Let x ∈ H and let p ∈ I be a rank one operator with p(x) = x. Then p = u + v for some u ∈ N ((La )r ) and some v ∈ R(La ). We have v = La1 (v1 ) + · · · + Lan (vn ) for some v1 , . . . , vn ∈ I. Now x = (u+v)(x) = u(x)+(a1 v1 +· · ·+an vn )(x) ∈ N (ar )+R(a). Hence N (ar )+R(a) = H and so δ(a) ≤ δ(La ). For the reverse inequality, suppose N (ar ) + R(a) = H. Let q ∈ B(H) be the orthogonal projection onto N (ar ). Define T : H n+1 → H by T (x, y1 , . . . , yn ) = q(x) + a1 (y1 ) + · · · + an (yn ). Note that T is surjective and so there exists a right inverse operator C : H → H n+1 . Write C(z) = (f (z), g1 (z), . . . , gn (z)) where f, g1 , . . . , gn ∈ B(H). Then for all u ∈ I we have u = (T ◦ C)u = (q ◦ f )u + (a1 ◦ g1 )u + · · · + (an ◦ gn)u ∈ N ((La )r ) + R(La ) We conclude that N ((La )r ) + R(La ) = I and so δ(La ) ≤ δ(a).

By appealing to adjoints we can obtain further equalities. Given an n-tuple a = (a1 , . . . , an ) we denote by a∗ = (a∗1 , . . . , a∗n ) the n-tuple of adjoint operators. Theorem 1.2. Let a = (a1 , . . . , an ) be an n-tuple of operators on a Hilbert space H. Let I be a two-sided ideal of B(H). Then (i) α(Ra , I) = α(a∗ , H) (ii) δ(Ra , I) = δ(a∗ , H) Proof. Note that the operator Ra : I → I is similar to the operator La∗ : I → I where similarity is provided by the involution τ : I → I, b → b∗ . Applying this similarity we have α(Ra , I) = α(La∗ , I) and δ(Ra , I) = δ(La∗ , I). The result now follows from Theorem 1.1. Remark 1.3. The ascent and descent of the right multiplication Ra behave somewhat differently to left multiplication. For example if a is the unilateral shift operator on 2 then δ(a, 2 ) = δ(La , B(2 )) = ∞ but δ(Ra , B(2 )) = 0. (This is noted in [1, Remark 2.3].) For a general Hilbert space operator, if ar+1 has closed range where r = α(a) then α(a, H) = δ(Ra , B(H)). The necessity of the closed range condition is illustrated by the self-adjoint operator a(ej ) = 21j ej on 2 . In this case

20

D. Kitson

α(a, 2 ) = 0 and a does not have closed range. It follows from the above theorem that δ(Ra , B(2 )) = ∞. For a single operator a ∈ B(H) with finite ascent and descent we have equalities α(a) = δ(a) = α(a∗ ) = δ(a∗ ) = α(La ) = δ(La ) = α(Ra ) = δ(Ra ). The following example shows that this is not always true in the multivariable case. Example. Let H be the Hilbert space with orthonormal basis (ei,j )∞ i,j=1 . Define a = (a1 , a2 ) where 1 ei−1,j if i > 1 and a2 (ei,j ) = j ei,j a1 (ei,j ) = 0 if i = 1 2 Then α(a) = δ(a) = 0 but α(a∗ ) = 0 and δ(a∗ ) = ∞. Theorem 1.4. Let a = (a1 , . . . , an ) and b = (b1 , . . . , bm ) be tuples of operators on a Hilbert space H. Let I be a two-sided ideal of B(H). (i) If α(a) > 0 and α(b∗ ) > 0 then α(a) + α(b∗ ) − 1 ≤ α((La , Rb ), I). (ii) If δ(a) > 0 and α(b) > 0 then δ(a) + α(b) − 1 ≤ δ((La , Rb ), I). Proof. (i) Suppose there exist non-zero elements x ∈ N (a)∩R(as ) and y ∈ N (b∗ )∩ ∗ t R((b ) ). Define the rank one operator p ∈ I by p(z) = z, yx for all z ∈ H. Now x= ai1 . . . ais (xi1 ...is ) for some xi1 ...is ∈ H. Also y = b∗j1 . . . b∗jt (yj1 ...jt ) for some yj1 ...jt ∈ H. For each i1 , . . . , is = 1, . . . , n and each j1 , . . . , jt = 1, . . . , m define the rank one operator pi1 ...is j1 ...jt ∈ I by pi1 ...is j1 ...jt (z) = z, yj1 ...jt xi1 ...is for all z ∈ H. Then we can verify that p = ai1 . . . ais pi1 ...is j1 ...jt bj1 . . . bjt . We have p ∈ N (La , Rb ) ∩ R((La , Rb )s+t ) and so s + t + 1 ≤ α(La , Rb ). It follows that α(a) + α(b∗ ) − 1 ≤ α(La , Rb ). (ii) Suppose there exists x ∈ N (b) ∩ R(bt ) with x = 1 and suppose there exists y ∈ H with y ∈ / N (as ) + R(a). Define the rank one operator q ∈ I by q(z) = z, xy for all z ∈ H. If δ(La , Rb ) ≤ s + t then N ((La , Rb )s+t ) + R(La , Rb ) = I. Thus we can write q = v+w for some v ∈ N ((La , Rb )s+t ) and some w ∈ R(La , Rb ). Now v(x) ∈ N (as ) and w(x) ∈ R(a). Thus y = q(x) = v(x) + w(x) ∈ N (as ) + R(a) which is a contradiction. We conclude that s + t + 1 ≤ δ(La , Rb ). It now follows that δ(a) + α(b) − 1 ≤ δ((La , Rb ), I). Corollary 1.5. Let a = (a1 , . . . , an ) and b = (b1 , . . . , bm ) be tuples of operators on a Hilbert space H. Let I be a two-sided ideal of B(H). Suppose s = α(a) = δ(a) < ∞ and t = α(b∗ ) = δ(b∗ ) < ∞. Then (i) α((La , Rb ), I) = δ((La , Rb ), I) = 0

if s = 0 or t = 0;

(ii) α((La , Rb ), I) = δ((La , Rb ), I) = s + t − 1

if s, t > 0.

Proof. By Theorem 1.1 we have α(La ) = δ(La ) = s and by Theorem 1.2, α(Rb ) = δ(Rb ) = t. (i) is clear so suppose s, t > 0. The operators in La commute with the operators in Rb and so from the argument in [8, Proposition 2.1], (La , Rb ) has finite ascent and descent at most s + t − 1. Now apply Theorem 1.4.


21

Lemma 1.6. Let s = (s1 , . . . , sn ) and t = (t1 , . . . , tm ) be tuples of linear mappings on a vector space X such that si tj = tj si for all i, j. Let st = (si tj )n,m i=1,j=1 . Then (i) α(st) ≤ max(α(s), α(t)) (ii) δ(st) ≤ max(δ(s), δ(t)) Proof. Suppose r = max(α(s), α(t)) < ∞. If x ∈ N (st) ∩ R((st)r ) then tj (x) ∈ N (s) ∩ R(sr ) = {0} for all j Hence x ∈ N (t) ∩ R(tr ) = {0}. We conclude that α(st) ≤ r. Considering transpose operators acting on the algebraic conjugate of X (and noting [8, Proposition 3.4]) we have δ(st) = α((st) ) = α(s t ) ≤ max(α(s ), α(t )) = max(δ(s), δ(t)). Theorem 1.7. Let a = (a1 , . . . , an) and b = (b1 , . . . , bm ) be tuples of operators on a m Hilbert space H. Let I be a two-sided ideal of B(H) and let Ma,b = (Mai ,bj )n, i=1,j=1 . Then (i) min(α(a), α(b∗ )) ≤ α(Ma,b , I) ≤ max(α(a), α(b∗ )) (ii) min(δ(a), α(b)) ≤ δ(Ma,b , I) ≤ max(δ(a), δ(b∗ )) Proof. By Lemma 1.6 we have α(Ma,b ) ≤ max(α(La ), α(Rb )) and δ(Ma,b ) ≤ max(δ(La ), δ(Rb )). Now Theorem 1.1 and Theorem 1.2 show the rightmost inequalities in (i) and (ii) hold. Suppose there are non-zero elements x ∈ N (a) ∩ R(as ) and y ∈ N (b∗ ) ∩ ∗ t R((b ) ). Define p ∈ I by p(z) = z, yx. Then p ∈ N (Ma,b ) ∩ R((Ma,b )r ) where r = min(s, t). It follows that min(α(a), α(b∗ )) ≤ α(Ma,b ). Suppose there exists x ∈ H with x ∈ / N (as ) + R(a) and y ∈ N (b) ∩ R(bt ) with y = 1. Define p ∈ I by p(z) = z, yx. Let r = min(s, t) and suppose N ((Ma,b )r ) + R(Ma,b) = I. Then p = v + w for some v ∈ N ((Ma,b )r ) and some w ∈ R(Ma,b ). Now v(y) ∈ N (as ) and w(y) = 0. Hence x = p(y) = v(y) ∈ N (as ). This is a contradiction and so min(s, t) < δ(Ma,b ). The result follows.

2. Browder spectrum of (La , Rb ) Throughout this section H is a complex Hilbert space and we consider multiplication operators acting on B(H) or on a norm ideal I of B(H). For a general reference on joint spectra we cite [11]. An n-tuple a = (a1 , . . . , an ) of operators on a Banach space X is left (resp. right) invertible if there exists b1 , . . . , bn ∈ B(X) with b1 a1 + · · · + bn an = I (resp. a1 b1 + · · · + an bn = I). Denote by I lef t the collection of left invertible tuples and by I right the collection of right invertible tuples. Recall the left and right spectra for a tuple of operators are / I left } σl (a) = {λ ∈ Cn : a − λ ∈ / I right } σr (a) = {λ ∈ Cn : a − λ ∈ An n-tuple a = (a1 , . . . , an ) is called upper semi-Fredholm if the column operator X → X n , x → (a1 (x), . . . , an (x)) has finite-dimensional kernel and closed range. An n-tuple a = (a1 , . . . , an ) is lower semi-Fredholm if the row operator X n →

22

D. Kitson

X, (x1 , . . . , xn ) → a1 (x) + · · · + an (x) has finite codimensional range. We denote by F + and F − respectively the collection of upper and lower semi-Fredholm tuples. The upper and lower Fredholm spectra are / F +} σe+ (a, X) = {λ ∈ Cn : a − λ ∈ σe− (a, X) = {λ ∈ Cn : a − λ ∈ / F −} An n-tuple is called Fredholm if it is both upper and lower semi-Fredholm. We will make use of the following formulae which can be found in [2]. σe+ ((La , Rb ), I) = σl (a) × σe− (b) ∪ σe+ (a) × σr (b) (2.1) − − + (2.2) σe ((La , Rb ), I) = σr (a) × σe (b) ∪ σe (a) × σl (b) We denote by B + the collection of upper semi-Fredholm tuples with finite ascent and by B − the collection of lower semi-Fredholm tuples with finite descent. Define σb+ (a) = {λ ∈ Cn : a − λ ∈ / B+} σb− (a) = {λ ∈ Cn : a − λ ∈ / B−} σb (a, X) = σb+ (a) ∪ σb− (a) In [8] it is shown that for commuting tuples σb is a compact-valued joint spectrum which satisfies a spectral mapping theorem. We note the following inclusions, σe+ (a) ⊆ σb+ (a) ⊆ σl (a) σe− (a) ⊆ σb− (a) ⊆ σr (a) Theorem 2.1. Let a = (a1 , . . . , an ) and b = (b1 , . . . , bm ) be tuples of operators on a complex Hilbert space H and let I be either B(H) or a norm ideal of B(H). Then σb ((La , Rb ), I) = S1 ∪ S2 where S1 = σl (a) × σb− (b) ∪ σb+ (a) × σr (b) S2 = σr (a) × σb+ (b) ∪ σb− (a) × σl (b) Proof. First we show the inclusion σb (La , Rb ) ⊇ S1 ∪S2 . Suppose λ ∈ / σb (La , Rb ). For simplicity we assume λ = 0 ∈ Cn+m . Then (La , Rb ) is Fredholm with finite ascent and finite descent. Using (2.1) and (2.2) we have four possible cases to consider: / (σr (a) × σl (b)) (i) 0 ∈ / (σl (a) × σr (b)) and 0 ∈ (ii) 0 ∈ / σl (a) × σr (b) and 0 ∈ (σr (a)\σe− (a)) × (σl (b)\σe+ (b)) (iii) 0 ∈ / σr (a) × σl (b) and 0 ∈ (σl (a)\σe+ (a)) × (σr (b)\σe− (b)) (iv) 0 ∈ (σr (a)\σe− (a)) × (σl (b)\σe+ (b)) and 0 ∈ (σl (a)\σe+ (a)) × (σr (b)\σe− (b)) If (i) holds then clearly 0 ∈ / S1 ∪ S2 . Suppose (ii) holds. Since 0 ∈ σr (a) we have δ(a) > 0. Since 0 ∈ σl (b) and b is upper semi-Fredholm we have α(b) > 0. Thus by Theorem 1.4, δ(a) + α(b) − 1 ≤ δ(La , Rb ) < ∞. From (ii) we have either 0∈ / σl (a) or 0 ∈ / σr (b). If 0 ∈ / σl (a) then α(a) = 0. But this forces α(a) = δ(a) = 0 which is a contradiction. Similarly if 0 ∈ / σr (b) then α(b) = δ(b) = 0 which is a contradiction. We conclude that (ii) cannot hold. A similar argument shows that


23

(iii) cannot hold. If (iv) holds then a and b are both Fredholm tuples with non-zero ascent and descent. Since b is Fredholm we have α(b∗ ) = δ(b) ([8, Proposition 3.5]). Applying Theorem 1.4 we see that a and b both have finite ascent and finite descent. Hence 0 ∈ / S1 ∪ S2 . This shows that σb (La , Rb ) ⊇ S1 ∪ S2 . / S1 ∪ S 2 . Next we show the inclusion σb (La , Rb ) ⊆ S1 ∪ S2 . Suppose λ ∈ Again we will assume λ = 0 ∈ Cn+m . Using (2.1) and (2.2) we see that (La , Rb ) is Fredholm. To show that (La , Rb ) has finite ascent and descent we consider four possibilities: (i) 0 ∈ / (σl (a) × σr (b)) and 0 ∈ / (σr (a) × σl (b)) (ii) 0 ∈ / σl (a) × σr (b) and 0 ∈ (σr (a)\σb− (a)) × (σl (b)\σb+ (b)) (iii) 0 ∈ / σr (a) × σl (b) and 0 ∈ (σl (a)\σb+ (a)) × (σr (b)\σb− (b)) (iv) 0 ∈ (σr (a)\σb− (a)) × (σl (b)\σb+ (b)) and 0 ∈ (σl (a)\σb+ (a)) × (σr (b)\σb− (b)) If (i) holds then we obtain one of the four conditions α(a) = δ(a) = 0

α(b∗ ) = δ(b∗ ) = 0

α(a) = δ(b∗ ) = 0

α(b∗ ) = δ(a) = 0

From Theorem 1.1 and Theorem 1.2, each condition gives α(La , Rb ) = δ(La , Rb ) = 0. If (ii) or (iii) holds then we have either α(a) = δ(a) = 0 or α(b∗ ) = δ(b∗ ) = 0. Again this implies α(La , Rb ) = δ(La , Rb ) = 0. If (iv) holds then both a and b∗ are Fredholm tuples with finite non-zero ascent and descent. By Corollary 1.5 we have α(La , Rb ) = δ(La , Rb ) < ∞. Hence 0 ∈ / σb (La , Rb ). This shows σb (La , Rb ) ⊆ S1 ∪ S 2 . n We use below the standard notation A ◦ B = { i=1 λi μi : λ ∈ A, μ ∈ B} for subsets A, B of Cn . Corollary 2.2. Let a = (a1 , . . . , an ) and b = (b1 , . . . , bn ) be commuting n-tuples of operators on a complex Hilbert space H and let I be B(H) or a norm ideal of B(H). Let E : I → I be the elementary operator E(u) = a1 ub1 + · · · + an ubn . Then σb (E, I) = σl (a) ◦ σb− (b) ∪ σb+ (a) ◦ σr (b) ∪ σr (a) ◦ σb+ (b) ∪ σb− (a) ◦ σl (b) Proof. Write E = p(La , Rb ) where p is the polynomial p(z1 , . . . , zn , w1 , . . . , wn ) = n z i i=1 wi . Applying the spectral mapping theorem for σb ([8, §4]) we obtain σb (E) = p(σb ((La , Rb ), I)) and so the result follows from Theorem 2.1. The Taylor–Browder spectrum of a commuting n-tuple a = (a1 , . . . , an ) is σT b (a) = acc (σT (a)) ∪ σT e (a) where σT is the Taylor spectrum, σT e is the essential Taylor spectrum and acc denotes the set of accumulation points. The Taylor–Browder spectrum of (La , Rb ) is readily deduced from [5], σT b ((La , Rb ), I) = (σT (a) × σT b (b)) ∪ (σT b (a) × σT (b)) Application of the spectral mapping theorem for σT b ([4]) yields σb (E, I) = (σT (a) ◦ σT b (b)) ∪ (σT b (a) ◦ σT (b))

24

D. Kitson

Combining this with Corollary 2.2 and noting inclusions we obtain the formula σb (E, I) = (σH (a) ◦ σb (b)) ∪ (σb (a) ◦ σH (b)) where σH = σl ∪ σr denotes the Harte spectrum. Acknowledgment Many thanks to Martin Mathieu for prompting this research and to the Analysis group at TCD for valuable discussions.

References [1] M. Burgos, A. Kaidi, M. Mbekhta, M. Oudghiri, The descent spectrum and perturbations, J. Operator Theory 56(2) (2006), 259–271. [2] A. Carrillo, C. Hern´ andez, Spectra of constructs of a system of operators, Proc. Amer. Math. Soc. 91(3) (1984), 426–432. [3] R.E. Curto, Spectral theory of elementary operators, in: M. Mathieu (ed.), Elementary operators and applications (Blaubeuren 1991), World Sci. Publ., River Edge, NJ (1992), 3–52. [4] R.E. Curto, A.T. Dash, Browder spectral systems, Proc. Amer. Math. Soc. 103(2) (1988), 407–413. [5] R.E. Curto, L. Fialkow, The spectral picture of [LA , RB ], J. Funct. Anal. 71 (1987), 371–392. [6] L. Fialkow, Essential spectra of elementary operators, Trans. Amer. Math. Soc. 267(1) (1981), 157–174. [7] R.E. Harte, Tensor products, multiplication operators and the spectral mapping theorem, Proc. Royal Irish Acad. 73A (1973), 285–302. [8] D. Kitson, Ascent and descent for sets of operators, Studia Math. 191(2) (2009), 151–161. [9] G. Lumer, M. Rosenblum, Linear operator equations, Proc. Amer. Math. Soc. 10(1) (1959), 32–41. [10] M. Mathieu, Elementary operators on prime C ∗ -algebras, I, Math. Ann. 284 (1989), 223–244. [11] V. M¨ uller, Spectral Theory of Linear Operators and Spectral Systems in Banach Algebras, Birkh¨ auser Verlag, 2007. Derek Kitson School of Mathematics Trinity College Dublin 2, Ireland e-mail: [email protected]

Approximation of Maps on C ∗ -algebras by Completely Contractive Elementary Operators Bojan Magajna p.n.

Abstract. Let E1 (A) be the closure in the point-norm topology of the set of all completely contractive elementary operators on a C ∗ -algebra A. If ψ ≤ φ p.n. p.n. are completely positive maps on A and φ ∈ E1 (A) , then ψ ∈ E1 (A) . A p.n. completely positive contraction φ on a von Neumann algebra R is in E1 (R) p.n. if and only if the normal and the singular part of φ are both in E1 (R) . Maps on R admitting pointwise approximation by sequences of elementary complete contractions may have additional properties that are not shared by p.n. all maps in E1 (R) . A specific example on B(H) is also studied. Mathematics Subject Classification (2000). Primary 46L07; Secondary 47B47. Keywords. Elementary operators, complete contractions, C ∗ -algebras, von Neumann algebras.

1. Introduction and notation Given a Banach algebra A, we may ask which operators on A can be approximated by elementary operators, either uniformly or pointwise. Clearly all such operators must preserve ideals of A (where by an ideal we mean a closed two-sided ideal) and for C ∗ -algebras this necessary condition is also sufficient for the pointwise approximation [2], [13]. Here by an operator we shall always mean a linear bounded operator. A variant of the above problem is to characterize all Banach or all C ∗ -algebras A such that every operator on A that preserves ideals can be approximated uniformly by elementary operators. Separable C ∗ -algebras with this property are characterized in [17] (they are just finite direct sums of homogeneous C ∗ algebras Partially supported by the Ministry of Science and Education of Slovenia. The author is grateful to Aleksej Turnˇsek and Milan Hladnik for their remarks concerning the paper.


25

26

B. Magajna

of finite type), but for nonseparable C ∗ -algebras and more general Banach algebras the problem has not yet been studied, as far as we know. Since the set of all elementary operators on a (unital) C ∗ -algebra A coincides with the range of the natural map from the algebraic tensor product A ⊗Z A (over the center Z of A) to A, this question is in some sense dual to the problem of characterizing all C ∗ algebras A for which the multiplication map is (completely) isometric when A ⊗Z A is equipped with the Haagerup (semi-)norm. The latter problem received much attention (see [8], [20], [6], [1], [14], [2] and the references there) and a definitive solution has been obtained recently by Somerset [21] and Archbold, Somerset and Timoney [3]. On a C ∗ -algebra every elementary operator ψ is a completely bounded map in the sense that ψcb = supn ψn < ∞, where for each n ∈ N we denote by ψn the map on Mn (A) obtained by applying ψ entrywise. So, in this case we may ask a sharper question: which complete contractions φ on A (that is, φcb ≤ 1) that preserve closed two-sided ideals, can be approximated pointwise by elementary complete contractions (that is, by completely contractive elementary operators)? We note that the approximation on singletons is always possible [15], the problem is the simultaneous approximation on more general finite subsets of a C ∗ -algebra. It turns out that this question is also related to C ∗ -tensor products of C ∗ -algebras [16]. (However, here we will not study the relation to tensor products of C ∗ -algebras and no knowledge of the theory of tensor products of C ∗ -algebras is needed to read this paper.) Since the Calkin algebra C(H) = B(H)/K(H) of a separable Hilbert space is algebraically simple, it is a consequence of the classical Jacobson density theorem that every linear operator φ on C(H) coincides on each finite-dimensional subspace V of C(H) with some elementary operator ψV . But if φcb < 1, in general ψV can not be chosen to be completely contractive; namely, as observed in [16], there is a lifting obstruction for such an approximation. Thus, the requirement that the approximands should be completely contractive is essential for the character of the approximation problem studied here. In Section 2 we will reformulate the above question and provide a different approach to a part of the main result of [16]. In Section 3 we will deduce some consequences for completely positive maps of the result of Section 2, needed later. In particular, for completely positive contracp.n. p.n. tions ψ ≤ φ on a C ∗ -algebra A we show that φ ∈ E1 (A) implies ψ ∈ E1 (A) , p.n. denotes the point-norm closure of the set E1 (A) of all elementary where E1 (A) operators on a C ∗ -algebra A that can be represented by tensors in A ⊗ A with the Haagerup norm at most 1. (By Theorem 2.1 below this is the same as the point-norm closure of all completely contractive elementary operators on A.) Using these we will show in Section 4 that a completely positive contraction p.n. on a von Neumann algebra R is in E1 (R) if and only if its normal and its p.n. singular part are both in E1 (R) . We will also note that if a map φ on R can be approximated pointwise by a sequence of completely contractive elementary

Approximation by Elementary Complete Contractions

27

operators, then its second adjoint φ∗∗ on R∗∗ is a module homomorphism over the center of R∗∗ , and that this property does not hold in general for all maps p.n. in E1 (R) . Recall that every bounded linear map φ on B(H) can be decomposed uniquely as φ = φsing + φnor , where φnor is normal (meaning weak* continuous) and φsing is singular (which is equivalent to φsing (K(H)) = 0). If φ is completely contractive, Voiculescu’s theorem [4], [23] can be used to approximate the singular part φsing by two-sided multiplications of the form x → axb, where a and b are contractions on H, and the approximation problem can be reduced to normal maps. It is well known that all normal completely bounded maps on B(H) (H separable) are of ∗ the form φ(x) = ∞ i=1 ai xbi , where ai and bi are elements of B(H) such that the two infinite columns a = (a1 , a2 , . . .)T and b = (b1 , b2 , . . .)T representbounded ∞ operators from H into H (in other words, the two sums a∗i ai and b∗i bi are convergent in the strong operator topology). Occasionally we shall denote such a map φ by a∗ b. But such maps do not always preserve compact operators: for example, if eij are the matrix units of B(H) (with respect to some orthonormal basis of H), the map x → ei1 xe1i sends the rank 1 projection e11 into the identity operator, which is not compact. In Section 5 we will consider a special class of normal complete contractions on B(H) that do preserve the ideal K(H) of compact operators and show that they can not always be approximated pointwise by sequences of elementary complete contractions. Since B(H) is not separable in p.n. and the question norm, this does not imply that such maps are not in E1 (B(H)) p.n. if all complete contractions on B(H) that preserve K(H) are in E1 (B(H)) is not answered completely in this paper. Of course, the approximation in the pointweak* topology is always possible in B(H). More generally it follows from results of Chatterjee and Smith [6] that each complete contraction on an injective von Neumann algebra R which preserves weak* closed two-sided ideals of R can be approximated by elementary complete contractions in the point-weak* topology. We shall denote by Φ the universal representation of a von Neumann (or ∗ a C -algebra) R. Then the weak* closure Φ(R) of Φ(R) (in B(K), where K is the Hilbert space of Φ) is naturally identified with the bidual R∗∗ of R. Each functional in R∗ can be extended uniquely to a normal functional on R∗∗ and the map Φ−1 : Φ(R) → R has the weak* continuous extension Φ−1 : R∗∗ → R (or to R if R is merely a C ∗ -algebra represented on a Hilbert space). Since Φ−1 is a ∗-homomorphism, the kernel of Φ−1 is a weak* closed ideal in R∗∗ , hence of the form P ⊥ R∗∗ for a central projection P ∈ R∗∗ . Since the map φ∗∗ on R∗∗ is weak* continuous, the composite φ := Φ−1 φ∗∗ is also weak* continuous and is the unique weak* continuous extension of φΦ−1 to a map R∗∗ → R. The normal and the singular part of φ are then given by φnor (x) = φ(P Φ(x)) and φsing (x) = φ(P ⊥ Φ(x)) (x ∈ R). More explanation on this can be found, e.g., in [12, Section 10.1].

28

B. Magajna

2. A preliminary result The following result is a reformulation of a part of [16, Theorem 2.1] (with the additional observation, that it suffices to consider the universal representations). We will give a proof, different from that in [16]. Theorem 2.1. Let φ be a complete contraction on a C ∗ -algebra A such that φ(J) ⊆ J for each closed two-sided ideal J of A. Then φ can be approximated pointwise by a net of elementary complete contractions ψk if and only if for each representation π : A → B(H) and all finite subsets {x1 , . . . , xn } of A and {x1 , . . . , xn } of π(A) (the commutant of π(A)) the inequality n n xi π(φ(xi )) ≤ xi π(xi ) (2.1) i=1

i=1

holds. Moreover, it suffices that (2.1) holds for the universal representation of A and the approximating operators ψk can be chosen to be of the form ψk (x) = ∗ ∗ a xb (a , b ∈ A) with a a ≤ 1, i,k i,k i,k i i,k i i,k i,k i bi,k bi,k ≤ 1 (that is, the Haagerup norms of tensors corresponding to the maps ψk are at most 1). Proof. Suppose first that φ is an elementary operator on A with φcb < 1. Let π : A → B(H) be a representation of A and φπ the elementary operator induced by φ on the weak* closure R of π(A). Since φπ cb is equal to the norm of the h tensor corresponding to φπ in the central Haagerup ∗ tensor product R ⊗Z R (see [6]), we have that φπ is of the form φπ (x) = j cj xdj (x ∈ R), where cj , dj ∈ R and the columns c := (c1 , . . . , cm )T and d := (d1 , . . . , dm )T have norms less than 1. Then for any xi ∈ A and xi ∈ π(A) (i = 1, . . . , n) we compute (denoting by x(m) the m × m diagonal matrix with x along the diagonal) that i xi π(φ(xi )) = i xi φπ (π(xi )) = i xi j c∗j π(xi )dj ∗ ( i xi π(xi ))(m) d ≤ c i xi π(xi )d = c ≤ i xi π(xi ). This proves (2.1) in the case φ is elementary with φcb < 1, the general case follows by an approximation. Suppose now that (2.1) holds for the universal representation of A. We will regard A as a C ∗ -subalgebra of its universal von Neumann envelope A = A∗∗ (and so regard the universal representation as the inclusion). So the condition (2.1) simply says that n n φ(xi )xi ≤ xi xi i=1

i=1 {x1 , . . . , xn }

of A , which means that for all finite subsets {x1 , . . . , xn } of A and φ extends to a contractive A -bimodule homomorphism φ˜ of the C ∗ -algebra AA generated by A and A inside B(K), where K is the Hilbert space of the universal representation of A. Since every normal state on A (that is, every state on A) is a vector state [12, Section 10.1], every finite subset of K is contained in


29

a cyclic subspace for A by [20, Lemma 2.3] and it follows that φ˜ is completely contractive by [20, Theorem 2.1 and Remark 2.2]. Then by an appropriate variant of the Wittstock extension theorem [5], [18, p. 116] φ can be extended to a completely contractive A -bimodule homomorphism ψ on B(K). By [8] the space CBA (B(K))A of all completely bounded A -bimodule homomorphisms on B(K) h

can be identified with the second dual (A ⊗ A)∗∗ of the Haagerup tensor product h

A ⊗ A completely isometrically and weak* homeomorphically by the map which h

extends the well-known complete isometry A ⊗ A → B(K). (Perhaps a simpler proof of this is in [7]: on the level of preduals the corresponding map is the result h

h

eh

eh

eh

of the following identifications: K∗ ⊗A B(K) ⊗A K = K∗ ⊗ A (K ⊗ K∗ ) ⊗A K = h

eh

h

eh

eh

(K∗ ⊗A K) ⊗ (K∗ ⊗A K) = A∗ ⊗ A∗ . Here ⊗ denotes the extended Haagerup tensor product and we have used some relations from [7].) It follows that ψ is the point-weak* limit of a net ψk of elementary complete contractions on B(K) of the form ψ (x) = k i ai,k xbi,k , where ai,k , bi,k ∈ A, for each k the sum is finite and ∗ ∗ i ai,k ai,k ≤ 1, i bi,k bi,k ≤ 1. Since such elementary operators map A into A and each functional in A∗ extends to a normal functional on B(K), it follows that the restrictions ψk |A converge to φ = ψ|A in the point-weak topology. Since the point-weak and the point-norm topology have the same continuous linear functionals, it follows by a well-known convexity argument that φ can be approximated by elementary complete contractions of the required form.

3. Pointwise approximation of completely positive maps by completely contractive elementary operators Proposition 3.1. Let φ and ψ be completely positive maps on a C ∗ -algebra A such p.n. that ψ ≤ φ (that is, φ − ψ is completely positive). If φ ∈ E1 (A) , then ψ ∈ p.n. E1 (A) . Proof. We shall use Theorem 2.1. Given a representation π of A and finite subsets {x1 , . . . , xn } of A and {x1 , . . . , xn } of π(A) , we denote by x the n×n matrix which has the first row (x1 , . . . , xn ) and the remaining rows all 0, and by x the matrix with the first column (x1 , . . . , xn )T and the remaining columns 0. We denote by π, φ and ψ also the amplifications of these maps to the matrix algebra Mn (A). Then, using the Schwarz inequality for completely positive maps, we compute that ∗ ∗ ∗ ∗ j π(ψ(xj ))xj 2 = i,j xi π(ψ(xi ) ψ(xj ))xj = x π(ψ(x) ψ(x))x ∗ ∗ ∗ ∗ ≤ x π(φ(x x))x ≤ x π(ψ(x x))x ∗ ∗ ∗ = i,j π(φ(xi xj ))x i xj ≤ i,j π(x∗i xj )x i xj 2 = j π(xj )xj , where for the last inequality we have used the condition (2.1) for the map φ and ∗ the subsets {x∗i xj : i, j = 1, . . . , n} and {x i xj : i, j = 1, . . . , n}.

30

B. Magajna

Given a map ψ on a C ∗ -algebra A and elements a, b ∈ A, we denote by aψb the map on A defined by (aψb)(x) = aψ(x)b. Lemma 3.2. Let ψ and θ be completely positive maps on a unital C ∗ -algebra A and let h, k ∈ A+ be such that h2 + k 2 ≤ 1. If for all t ∈ (0, 1) the two maps ht ψht and p.n. p.n. then φ := hψh + kθk is also in E1 (A) . k t θkt are in E1 (A) Proof. Let t ∈ (0, 1). For each x ∈ A we may write 1−t ht ψ(x)ht h 0 . φ(x) = h1−t k 1−t 0 k t θ(x)kt k 1−t If ψk and θk are nets of maps in E1 (A) converging pointwise to ht ψht and k t θkt , respectively, then the elementary operators φk := h1−t ψk h1−t + k1−t θk k 1−t converge pointwise to φ. Further, φk cb ≤ h2(1−t) + k2(1−t) , so now it suffices to observe that lim supt→0 h2(1−t) + k 2(1−t) ≤ 1. This follows from the relations limt→0 h2(1−t) − h2 = 0 = limt→0 k 2(1−t) − k 2 , which can be proved by the functional calculus. Lemma 3.3. Let ψ be a completely positive contraction on a unital C ∗ -algebra A p.n. p.n. and h ∈ A, 0 ≤ h ≤ 1. Suppose that hψh ∈ E1 (A) . Then ht ψht ∈ E1 (A) for p.n. for any continuous function f all t > 0; more generally, f (h)∗ ψf (h) ∈ E1 (A) on the spectrum of h such that f ≤ 1 and (if h is not invertible) f (0) = 0. p.n.

Proof. Since hψh ∈ E1 (A) , hψh satisfies the condition (2.1) of Theorem 2.1, hence for each positive constant c ∈ R the map θc := (h + c)−1 hf (h)∗ ψf (h)h(h + c)−1 satisfies the following condition: for every unital representation π of A and every finite subsets {x1 , . . . , xn } of A and {x1 , . . . , xn } of π(A) the inequality ≤ κ π(θ (x ))x π(x )x c i c i i i i

i

holds, where κc = f (h)h(h + c) . This means that the map i π(xi )xi → ∗ ˜ i π(θc (xi ))xi extends to a bounded map θc on the C -algebra π(A)π(A) . Since ψ is completely positive, it is not hard to verify that ∗ ˜ θc π(xi )xi π(xi )xi ≥ 0, −1 2

i

i

hence θ˜c is positive. A similar argument shows that θ˜c is completely positive, so θ˜c cb = θ˜c (1) ≤ θc (1) = f (h)h(h + c)−1 2 ≤ 1. Therefore ≤ π(θ (x ))x π(x )x c i i i i i

i

(xi )

and representation π as in Theorem 2.1, hence for any finite sets (xi ) and p.n. θc ∈ E1 (A) . This holds for every choice of c, so it suffices now to observe that θc tends uniformly to f (h)∗ ψf (h) as c → 0 since limc→0 f (h)h(h + c)−1 − f (h) = 0 by the functional calculus.


31

With a similar proof as that of Lemma 3.3 we can show the following proposition. Proposition 3.4. Let A be a unital C ∗ -algebra. If a completely positive map φ is p.n. in E1 (A) , then φ is in the pointwise closure of a net of elementary completely positive contractions σk . If, in addition, φ is unital, σk can be chosen to be unital. Proof. Let h = φ(1). If h is invertible, we consider the unital completely positive map ψ = h−1 φh−1 and show that it satisfies (2.1) by using complete positivity (in the same way as in the proof of Lemma 3.3). So by Theorem 2.1 there is a net ψk = a∗k bk in E1 (A) converging pointwise to ψ such that ak , bk ≤ 1. Since ak − bk 2 = (ak − bk )∗ (ak − bk ) ≤ 21 − Re(a∗ b) → 0,

(3.1)

the net of completely positive contractions θk = a∗k ak also converges pointwise to ψ. Hence the net of maps σk := hθk h = c∗k ck , where ck := ak h, converges to hψh = φ. If φ(1) = 1,we may replace each σk by the unital map τk (x) = σk (x) + 1 − c∗k ck x 1 − c∗k ck . In general (if h is not invertible), we consider first the maps (1−t)φ+t id (where id is the identity map on A) and then let t → 0. p.n.

4. Stability of maps in E1 (R) under the decomposition into the singular and the normal part The following lemma is perhaps well known. Lemma 4.1. Let φ : R → S be a completely positive map between von Neumann algebras and denote φ(1) = h2 , where h ≥ 0. Then there exists a unique completely positive map ψ : R → S such that ψ(1) is the range projection p of h and φ(x) = hψ(x)h for all x ∈ R. Moreover, ψ is normal (singular) if and only if φ is normal (singular). Proof. We may assume that φcb = 1, so that h = 1. The sequence of completely positive maps −1 −1 1 1 φ(x) h + (n ∈ N) ψn (x) = h + n n is bounded since ψn cb = ψn (1) ≤ 1, hence it has a limit point ψ in the pointweak* topology. From φ(x) = (h+ n1 )ψn (x)(h+ n1 ) we conclude that φ(x) = hψ(x)h for all x ∈ R. Since the sequence ψn (1) = (h + n1 )−1 h2 (h + n1 )−1 converges strongly to the range projection p of h, we have that ψ(1) = p. This proves the existence of ψ. The uniqueness of ψ is a consequence of the fact that the range of ψ must be contained in pSp (since 0 ≤ ψ(x) ≤ ψ(1) = p if 0 ≤ x ≤ 1). Namely, if ψ˜ is another such map, then the difference θ := ψ˜ − ψ satisfies hθ(x)h = φ(x) − φ(x) = 0. But, since θ(x) = pθ(x)p and the operator h = ph = hp is injective on the range of p, this implies that θ(x) = 0 for all (positive) x ∈ R. We shall now use the notation from the last paragraph of the Introduction. From φ = hψh we have that φ = hψh, hence, if φ is normal, then hψ(x)h = φ(x) =

32

B. Magajna

φ(P Φ(x)) = hψ(P Φ(x))h for all x ∈ R. Since ψ and ψ both have their ranges contained in pSp, this implies that ψ(x) = ψ(Φ(x)P ) (x ∈ R), so ψ is normal. The converse and the statement about singular maps is proved similarly. Corollary 4.2. A completely positive contraction φ on a von Neumann algebra R p.n. if and only if its normal part φnor and its singular part φsing are is in E1 (R) p.n. both in E1 (R) . p.n.

p.n.

Proof. If φ ∈ E1 (R) , Proposition 3.1 implies that φnor , φsing ∈ E1 (R) . To prove the converse, by Lemma 4.1 we write φnor = hψh and φsing = kθk where ψ is normal, θ is singular, both are completely positive contractions on R and h, k ∈ R+ satisfy h2 = φnor (1), k 2 = φsing (1), hence h2 + k 2 = φ(1) ≤ 1. Since φnor p.n. and φsing are in E1 (R) , it follows now from Lemma 3.3 that ht ψht and k t θk t p.n. p.n. are in E1 (R) for all t > 0. Then Lemma 3.2 implies that φ ∈ E1 (R) . The author does not know if Corollary 4.2 can be extended to all completely contractive maps. Note, however, that if φ is a point-norm limit of a sequence of elementary operators (or more generally, of a sequence of normal maps) φk on a von Neumann algebra R, then φ is necessarily normal. Indeed, for each ω in the predual R∗ of R the sequence (ω ◦ φk ) in R∗ converges weakly to ω ◦ φ, but R∗ is weakly sequentially complete [22, 5.2], so ω ◦ φ ∈ R∗ and φ is normal. Here is a somewhat related result. Proposition 4.3. If φ is a bounded linear map on a von Neumann algebra R which preserves norm closed two-sided ideals of R, then φnor and φsing also preserve such ideals. Proof. We shall use again the notation from the last paragraph of the Introduction. If J is a norm closed ideal in R, then J ∗∗ coincides with the weak* closure of Φ(J) in R∗∗ , hence is of the form J ∗∗ = pR∗∗ for a central projection p ∈ R∗∗ . From φ(J) ⊆ J we have that φ∗∗ (J ∗∗ ) ⊆ J ∗∗ , hence φ∗∗ (py) = pφ∗∗ (py) for all y ∈ R∗∗ . Let x ∈ J and apply the last equality to y = P Φ(x) = P pΦ(x) = pP Φ(x) to conclude that φnor (x) = φ(P Φ(x)) = Φ−1 (φ∗∗ (pP Φ(x))) = Φ−1 (pφ∗∗ (pP Φ(x))) ∈ Φ−1 (J ∗∗ ) ⊆ J, since Φ−1 is weak* continuous and Φ−1 (Φ(J)) = J. Then φsing (J) ⊆ J since φsing = φ − φnor . Proposition 4.4. If a bounded linear map φ on a von Neumann algebra R is a point-norm limit of a sequence of elementary operators on R then φ∗∗ is a module homomorphism over the center of R∗∗ . This conclusion does not hold necessarily p.n. if we assume only that φ ∈ E1 (R) . Proof. If φk is a sequence of elementary operators converging pointwise to φ, then for each functional ρ ∈ R∗ the sequence (φ∗k (ρ)) converges to φ∗ (ρ) ∈ R∗ in the weak* topology, hence, since R is a Grothendieck space [19], the convergence is ∗∗ in the weak topology of R∗ . This means that the maps φ∗∗ converge to k on R ∗∗ ∗∗ φ in the point-weak* topology. Note that each φk is an elementary operator on


33

R∗∗ , hence a module map over the center Z of R∗∗ , and that ρz ∈ R∗ for each z ∈ R∗∗ and ρ ∈ R∗ . (Here ρz is defined by (ρz)(x) = ρ(zx), where ρ is the unique weak* continuous extension of ρ to a normal functional on R∗∗ , and then ρz can be regarded as a normal functional on R∗∗ .) For each y ∈ R∗∗ and z ∈ Z we compute that ∗∗ ∗∗ ∗∗ φ∗∗ (zy), ρ = lim φ∗∗ k (zy), ρ = lim zφk (y), ρ = φ (y), ρz = zφ (y), ρ. k→∞

k→∞

Thus, φ∗∗ is a Z-module map. To show that the approximation by general nets is not sufficient for the above conclusion, let ωξk be a net of vector states on B(H) (ξk a unit vector in H) weak* converging to a non-normal state ω (by a result of Glimm [10] the weak* closure of vector states on B(H) consists precisely of states of the form tρ + (1 − t)θ, where t ∈ [0, 1], ρ is a vector state and θ is a state annihilating K(H)). Let x0 ∈ K(H) be the rank 1 operator η⊗η∗ , where η ∈ H is a unit vector. Then the net (φk ) of complete contractions on B(H) defined by φk (x) := ωξk (x)x0 (x ∈ B(H)) converges pointwise to the map φ defined by φ(x) = ω(x)x0 . Note that each φk is an elementary operator (which acts as x → (η ⊗ ξk∗ )x(ξk ⊗ η∗ )). Nevertheless, φ∗∗ is not a module map over the center Z of B(H)∗∗ . To see this, we shall again use the notation from the last paragraph of the Introduction, with R = B(H). If φ∗∗ were a Z-module map, then in particular φ∗∗ (P y) = P φ∗∗ (y) for all y ∈ B(H)∗∗ , hence the map φ = Φ−1 φ∗∗ would satisfy φ(P y) = Φ−1 (P φ∗∗ (y)) = Φ−1 (φ∗∗ (y) = φ(y). But then φ would be weak* continuous on B(H) [12, p. 721], a contradiction.

5. An example on B(H) In this section let H be a separable Hilbert space and K(H) the ideal of compact operators in B(H). As noted already in the Introduction, a general normal complete contraction on B(H) does not necessarily preserve the ideal K(H) and therefore can not be approximated pointwise in norm by elementary operators. However, for every orthogonal sequence of projections pi ∈ B(H) the completely positive contraction φ defined by ∞ pi xpi (x ∈ B(H)) (5.1) φ(x) = i=1

preserves K(H) for φ is just the block-diagonal compression, that is, it sends x n to the direct sum of operators pi xpi . Although the finite partial sums i=1 pi xpi usually do not converge in norm to the infinite sum in (5.1), this does not imply that φ can not be approximated pointwise in norm by elementary complete contractions. In fact, if the ranks of the projections pi are bounded, the range of φ p.n. is contained in a nuclear C ∗ algebra and φ ∈ E1 (B(H)) by [16]. On the other hand, if the ranks of the projections pi (are finite, but) increase to infinity (or, if infinitely many of the pi ’s have infinite rank), then the nuclearity argument does not apply. For simplicity we assume that pi = 1, so that φ is unital. The range

34

B. Magajna

of φ is a von Neumann algebra, which we denote by Bφ , and φ is an idempotent Bφ -bimodule map. We suspect that such maps are critical for the approximation problem on B(H) in the sense that if all maps of the form (5.1) can be approximated pointwise by completely contractive elementary operators then the same holds for all completely positive contractions that preserve K(H). We show first that, although φ is a Bφ -bimodule map, φ in general can not be approximated pointwise by completely contractive elementary operators which are Bφ -module maps. For this, we need the following finite-dimensional lemma. Lemma 5.1. Let M := Mn (C)n be embedded diagonally into N := Mn (Mn (C)), denote A := M ∼ = ∞ (n) (the commutant of M in N ) and let φ : N → M be the projection φ([xij ]) = ⊕nj=1 xjj . With (eij ) the usual matrix units in Mn (C), let y be the unitary element in N defined by y = [eji ] (that is, the entry on the position (i, j) is eji ).

(5.2)

For each m ∈ N denote by dm,n the distance of φ(y) to the set Sm := {ψ(y) : ψ ∈ m EUCPm M (N )}, where EUCPM (N ) is the set of all elementary unital completely positive M -bimodule maps of the form m m ψ(x) = a∗k xak , where ak ∈ A and a∗k ak = 1. (5.3) k=1

k=1

Then for each fixed m lim dm,n = 1.

n→∞

Proof. Given ψ of the form (5.3), noting that each ak is a diagonal matrix, ak = diag(αkj 1) (αkj ∈ C), we compute that the matrix ψ(y) is m αki eji αkj . ψ(y) = k=1 m

Thus, denoting for each i by αi ∈ C

the vector αi = (α1i , . . . , αmi ), we have

δ := ψ(y) − φ(y) = [(αi , αj − δij )eji ] .

(5.4)

From ψ(1) = 1 we compute that αi , αi = 1 for all i = 1, . . . , n. From (5.4) we have that |αi , αj | ≤ δ if i = j, hence αi − αj 2 = 2(1 − Reαi , αj ) ≥ 2(1 − δ) =: r2 , if i = j, i, j ∈ {1, . . . , n}. (5.5) This implies that any two of the n balls with centers αi and radius r/2 in Cm have at most one common point. Since all these balls are contained in the ball of Cm with center 0 and radius 1 + 2r , we conclude by considering the Euclidean volume that n( r2 )2m+1 ≤ (1 + 2r )2m+1 , hence from (5.5) 1 −2 r2 ≥ 1 − 2 n 2m+1 − 1 δ =1− . 2 1

Since this holds for all ψ, dm,n ≥ 1 − 2(n 2m+1 − 1)−2 , hence (since dm,n ≤ 1) dm,n necessarily tends to 1 as n → ∞.


35

For notational simplicity we shall consider now only a special kind of map of the form (5.1). Given a (strictly) increasing sequence (ni ) in N, let H = ni ni ⊕∞ and consider the diagonal unital embeddings i=1 (C ) B :=

∞

Mni (C)ni ⊆

i=1

∞

Ni ⊆ B(H), where Ni = Mni (Mni (C)).

(5.6)

i=1

For each i let yi ∈ Ni := Mni (Mni (C)) be the unitary defined by (5.2), so that y := ⊕∞ i=1 yi

(5.7)

is a unitary in B(H). Finally, let φ be the projection of B(H) onto B, so that φ(x) =

ni ∞

pik xpik (x ∈ B(H)),

(5.8)

i=1 k=1

where pik is the unit of the kth summand Mni (C) of Mni (C)ni inside Ni (a projection in B(H)). Note that B = imφ and A := B =

ni ∞

Cpik = the center of B.

i=1 k=1

Proposition 5.2. The inequality φ(y)−ψ(y) ≥ 1 holds for every unital completely positive map ψ on B(H) of the form ψ(x) =

m

a∗j xaj , where aj ∈ A and m ∈ N.

(5.9)

j=1

Further, φ can not approximated pointwise by elementary complete contractions be m of the form x → j=1 aj xbj , where all aj or all bj are in A. Proof. The first statement follows from Lemma 5.1 by considering the restrictions of φ to subalgebras Ni . Then the second statement follows by noting that the estimate (3.1) would allow us to replace completely contractive elementary approximands by unital completely positive elementary approximands as in the proof of Proposition 3.4. It can be shown by methods of [20] or [14] that all completely contractive elementary left B-module homomorphisms on B(H) are of the form x → j aj xbj , where aj ∈ B = A and bj ∈ B(H), hence by Proposition 5.2 the map φ can not be approximated pointwise in norm by such homomorphisms. The question of whether the conditional expectation from B(H) onto an atomic maximal abelian self-adjoint subalgebra A0 of B(H) can be approximated pointwise in norm by completely contractive A0 -bimodule homomorphisms, turns out to be equivalent to the well-known paving problem (hence, to the Kadison– Singer problem), but we shall not present a proof here.

36

B. Magajna

Lemma 5.3. Let E : B(H) → A be the diagonal projection, θ a unital completely positive map on B(H) of the form θ(x) =

m

b∗j xbj , where bj ∈ B(H) and m ∈ N,

j=1

set b := (b1 , . . . , bm ) and a := (E(b1 ), . . . , E(bm ))T =: (a1 , . . . , am )T . If θ(y) − φ(y) ≤ 1/4 (where y and φ are as above, defined by (5.7) and (5.8)), then a−b > 1/8. T

Proof. Assume the contrary, that a − b ≤ 1/8 and define a map ψ on B(H) by (5.9) with aj = E(bj ). By the Schwarz inequality for completely positive maps m m ∗ ∗ ∗ a a= E(bj ) E(bj ) ≤ E bj bj = 1. j=1

j=1

∗

∗

It follows (since θ(y) = b yb and ψ(y) = a ya) φ(y) − ψ(y) ≤ φ(y) − θ(y) + b∗ yb − a∗ ya ≤ 14 + 2b − a ≤ 12 .

(5.10)

Further, 1 − a∗ a = b∗ b − a∗ a ≤ 2b − a ≤ √ 1/4. Define √ a unital completely positive map ψ0 on B(H) by ψ0 (x) = ψ(x) + 1 − a∗ ax 1 − a∗ a. Then ψ0 − ψcb ≤ 1 − a∗ a ≤ 1/4, hence (5.10) implies that φ(y) − ψ0 (y) ≤ 3/4, which contradicts Proposition 5.2. Proposition 5.4. The map φ on B(H) defined by (5.8) cannot be approximated pointwise by any sequence of elementary complete contractions. Proof. Suppose the contrary, that a sequence (θk ) of elementary complete contractions on B(H) converges pointwise to φ. By a simple adjustment of coefficients (as in the proof of Proposition 3.4) we may assume the maps θk are unital k that ∗ and completely positive of the form θk (x) = m b xb k,j , hence the columns k,j j=1 bk := (bk,1 , . . . , bk,mk )T ∈ B(H)mk satisfy b∗k bk = 1. Let N = ∪∞ j=1 Nj be a partition of N into a sequence of infinite subsets Nj and for each j let Pj := pi , i∈Nj

where for each i the projection p ∈ B(H) is the unit of Ni (Ni is defined in (5.6)). k→∞ Then φ(x) − Pj θk (x)Pj = Pj (φ(x) − θk (x))Pj −→ 0 for each x ∈ Pj B(H)Pj . If E : B(H) → A is the diagonal projection, then E|Pj B(H)Pj is the diagonal projection onto APj and E(Pj xPj ) = E(x)Pj since Pj ∈ A. Applying Lemma 5.3 to the restriction φ|P1 B(H)P1 (with P1 y instead of the unitary y used in the lemma) it follows that 1 (5.11) P1 bk P1 − E(bk )P1 > 8 i


37

for all large enough k, say, for k ≥ k1 . Now recall that (since A = B is abelian, hence equal to the weak* closure of a union of an increasing net of finite-dimensional subalgebras) E (= the composition of the conditional expectation φ : B(H) → B followed by the central trace B → A) can be approximated in the point-weak* topology by convex combinations of maps of the form x → uxu∗ , where u ∈ B are unitary [12, p. 523 and p. 571]. So, it follows from (5.11) that there exists a unitary u1 ∈ P1 B such that P1 bk1 P1 − u1 bk1 u∗1 P1 > 1/8, that is P1 (bk1 u1 − u1 bk1 )P1 >

1 . 8

Similarly, applying Lemma 5.3 to the restriction φ|P2 B(H)P2 , there exists k2 > k1 such that P2 bk P2 − E(bk )P2 > 1/8 for all k ≥ k2 and consequently there exists a unitary u2 ∈ P2 B such that P2 (bk2 u2 − u2 bk2 )P2 >

1 . 8

Continuing in this way, we find a subsequence (bkj ) of (bk ) and a sequence of unitary elements uj ∈ Pj B such that Pj (bkj uj − uj bkj )Pj > Then u :=

n j=1

1 . 8

uj is a unitary in B and for each i

bki u − ubki ≥ Pi (bki u − ubki )Pi = Pi (bki uPi − Pi ubki )Pi = Pi (bki ui − ui bki )Pi > 18 . Since bki u − ubki 2 = (u∗ b∗ki − b∗ki u∗ )(bki u − ubki ) = 2 − b∗ki u∗ bki u − u∗ b∗ki ubki ≤ (u − b∗ki ubki )∗ u + u∗ (u − b∗ki ubki ) = 2u − b∗ki ubki , it follows that 1 . 2 · 82 must converge to φ(u) = u as i → ∞

u − b∗ki ubki ≥ But this is a contradiction since b∗ki ubki (because u ∈ B).

Elliott [9] proved that pointwise convergence to the identity of a sequence of completely positive maps on a W ∗ -algebra implies uniform convergence. Our initial proof of Proposition 5.4 was based on techniques of [9]. Later we have found a simpler proof presented above, for which the inspiration of constructing the appropriate unitary u comes from [11].

38

B. Magajna

References [1] P. Ara and M. Mathieu, On the central Haagerup tensor product, Proc. Edinburgh Math. Soc. 37 (1994), 161–174. [2] P. Ara and M. Mathieu, Local multipliers of C ∗ -algebras, Springer Monographs in Math., Springer-Verlag, Berlin, 2003. [3] R.J. Archbold, D.W.B. Somerset and R.M. Timoney, On the central Haagerup tensor product and completely bounded mappings of a C ∗ -algebra, J. Funct. Anal. 226 (2005), 406–428. [4] W.B. Arveson, Notes on extensions of C ∗ -algebras, Duke. Math. J. 44 (1977), 329– 355. [5] D.P. Blecher and C. Le Merdy, Operator algebras and their modules, L.M.S. Monographs, New Series 30, Clarendon Press, Oxford, 2004. [6] A. Chatterjee and R.R. Smith, The central Haagerup tensor product and maps between von Neumann algebras, J. Funct. Anal. 112 (1993), 97–120. [7] E.G. Effros and R. Exel, On multilinear double commutation theorems, Operator algebras and appl., Vol. 1, LMS Lecture Note Series, Vol. 135, Cambridge Univ. Press, Cambridge, 1988, pp. 81–94. [8] E.G. Effros and A. Kishimoto, Module maps and Hochschild–Johnson cohomology, Indiana Univ. Math. J. 36 (1987), 257–276. [9] G.A. Elliott, On the convergence of a sequence of completely positive maps to the identity, J. Austral. Math. Soc. Ser. A 68 (2000), 340–348. [10] J. Glimm, A Stone-Weierstrass theorem for C ∗ -algebras, Ann. of Math. 72 (1960), 216–244. [11] B.E. Johnson and S.K. Parrott, Operators commuting with a von Neumann algebra modulo the set of compact operators, J. Funct. Anal. 11 (1972), 39–61. [12] R.V. Kadison and J.R. Ringrose, Fundamentals of the theory of operator algebras, Vols. 1, 2, Academic Press, London, 1983, 1986. [13] B. Magajna, A transitivity theorem for algebras of elementary operators, Proc. Amer. Math. Soc. 118 (1993), 119–127. [14] B. Magajna, The Haagerup norm on the tensor product of operator modules, J. Funct. Anal. 129 (1995), 325–348. [15] B. Magajna, A transitivity problem for completely bounded mappings, Houston J. Math. 23 (1997), 109–120. [16] B. Magajna, Pointwise approximation by elementary complete contractions, Proc. Amer. Math. Soc. 137 (2009), 2375–2385. [17] B. Magajna, Uniform approximation by elementary operators, Proc. Edinburgh Math. Soc. 52 (2009), 731–749. [18] V.I. Paulsen, Completely bounded maps and operator algebras, Cambridge Studies in Advanced Mathematics 78, Cambridge University Press, Cambridge, 2002. [19] H. Pfitzner, Weak compactness in the dual of a C ∗ -algebra is determined commutatively, Math. Ann. 298 (1994), 349–371. [20] R.R. Smith, Completely bounded module maps and the Haagerup tensor product, J. Funct. Anal. 102 (1991), 156–175.


39

[21] D.W.B. Somerset, The central Haagerup tensor product of a C ∗ -algebra, J. Operator Theory 39 (1998), 113–121. [22] M. Takesaki, Theory of operator algebras I, Springer-Verlag, New York, 1979. [23] D.V. Voiculescu, A non-commutative Weyl–von Neumann theorem, Rev. Roum. Math. Pures Appl. 21 (1976), 97–113. Bojan Magajna Department of Mathematics University of Ljubljana Jadranska 21 1000 Ljubljana, Slovenia e-mail: [email protected]

Some Not-quite-elementary Operators Peter Rosenthal Abstract. Necessary and sufficient conditions are given that an operator equation of the form AXB+CY D = Z have a solution (X, Y ) for every operator Z. Mathematics Subject Classification (2000). Primary 47A62; Secondary 47B49, 15A24. Keywords. Operator equations, matrix equations.

Let B(H) denote the algebra of all bounded linear operators on the complex Hilbert space H. An elementary operator is a mapping of the form n X→ Ai XBi , i=1

where the {Ai } and the {Bi } are in B(H). The “not-quite-elementary operators” of the title are mappings from the direct sum of n copies of B(H) into B(H) of the form n (X1 , X2 , . . . , Xn ) → Ai Xi Bi i=1

where the {Ai } and the {Bi } are in B(H). Such operators appear to have been first studied in [1]. This expository paper discusses some of the results of [1] and presents a more direct proof of what might be regarded as the main theorem of [1]. (See Theorem 2 below.) Elementary operators originated from the study of the Sylvester–Rosenblum equation AX − XB = Z. (See [2] for a discussion of the many known results about this equation and of the wide variety of applications of those results.) Don Hadwin, Eric Nordgren, and I got interested in the study of “not-quite-elementary operators” because of our observation about the operator equation AX − Y B = Z in the finite-dimensional case. The surprising theorem is the following: Theorem 1. [1] For square complex matrices A and B, the mapping (X, Y ) → AX − Y B is onto if and only if at least one of the mappings X → AX or Y → Y B is onto.


41

42

P. Rosenthal

Proof. Suppose that the map (X, Y ) → AX − Y B is onto. If the map Y → Y B is not onto, then B has nullspace. The proof will be completed by showing that the map X → AX is onto in this case. Let f be a vector other than 0 such that Bf = 0. If X → AX is not onto, then A itself is not onto. Choose a g that is not in the range of A and pick any matrix Z such that Zf = g. Then Z cannot be of the form AX − Y B, since (AX − Y B)f = AXf , which is not Zf since g is not in the range of A. The same theorem holds in the infinite-dimensional case. In fact, a more general result was established in [1]. We rely on the following basic facts about operators. Recall that an operator is bounded below if and only if it has a left inverse. We will also use the obvious fact that an operator has a right inverse if and only if its adjoint has a left inverse. If the operator A is not bounded below, then, by definition, there exists a sequence {fn } of unit vectors such that {Afn} → 0. It follows by an elementary computation that goes back to Dixmier [3] (which can also be found in [4] and in [5]) that the sequence {fn } can be taken to be orthonormal. These elementary facts are all we need to prove the general theorem in the case of two summands. If the mapping (X1 , X2 ) → A1 X1 B1 + A2 X2 B2 is onto, then clearly (A1 A2 ) takes H ⊕ H onto H and taking adjoints shows that the same is true for (B1∗ B2∗ ). The theorem is the following: Theorem 2. [1] The mapping (X1 , X2 ) → A1 X1 B1 + A2 X2 B2 is onto if and only if a) both (A1 A2 ) and (B1∗ B2∗ ) take H ⊕ H onto H, and b) both of the operators in at least one of the following pairs are onto: {A1 , B1∗ }, {A2 , B2∗ }, {A1 , A2 }, and {B1∗ , B2∗ }. Proof. First suppose that the map defined by Φ(X1 , X2 ) = A1 X1 B1 + A2 X2 B2 is onto. Then a) clearly holds, so we must establish b). If neither of the elements of the set {B1∗ , A2 } is onto, then their adjoints are not bounded below, so there exist orthonormal sequences {fn } and {gn } such that {B1 fn } → 0 and {A∗2 gn } → 0. Then, for every X1 and X2 , ((A1 X1 B1 + A2 X2 B2 )fn , gn ) = (A1 X1 B1 fn , gn ) + (X2 B2 fn , A∗2 gn ) converges to 0 as n approaches infinity. On the other hand, since {fn } and {gn } are orthonormal sequences, there is an operator Z such that Zfn = gn for every n. Such an operator cannot be in the range of Φ since (Zfn , gn ) = 1 for all n. Hence at least one of the operators in the set {B1∗ , A2 } is onto. In exactly the same way, it can be established that at least one of the operators in the set {B2∗ , A1 } is onto. To get the conclusion of part b) of the theorem we now simply note the following: if at least one of the operators in each of the sets {B1∗ , A2 } and {B2∗ , A1 } is onto, then both of the operators in at least one of the sets {A1 , B1∗ }, {A2 , B2∗ }, {A1 , A2 }, {B1∗ , B2∗ } are onto. For the converse, it must be shown that conditions a) and b) imply that Φ is onto. Suppose then that a) holds. If both A1 and B1∗ are onto, then there exist

Some Not-quite-elementary Operators

43

operators C1 and D1 such that A1 C1 = I and D1 B1 = I. Hence, for any operator X, it follows that A1 (C1 XD1 )B1 = X, so Φ is onto. The same proof works if both A2 and B2∗ are onto. Suppose now that both of A1 and A2 are onto. Then there exist operators E1 and E2 such that A1 E1 = I and A2 E2 = I. Now, (B1∗ B2∗ ) is onto, so there exist operators F1 and F2 such that B1∗ F1 + B2∗ F2 = I. Given any Z then, let X1 = E1 ZF1∗ and X2 = E2 ZF2∗ . It follows that Φ(X1 , X2 ) = A1 (E1 ZF1∗ )B1 + A2 (E2 ZF2∗ )B2 = ZF1∗ B1 + ZF2∗ B2 = Z(F1∗ B1 + F2∗ B2 ) = Z. Thus in this case Φ is onto. The case where B1∗ and B2∗ are both onto is established in exactly the same way after taking adjoints. The fact that Theorem 1 holds for operators on Hilbert space is a special case of Theorem 2. Corollary 3. [1] For A and B operators on Hilbert space, the mapping (X, Y ) → AX − Y B is onto if and only if at least one of the mappings X → AX or Y → Y B is onto. Proof. In the notation of Theorem 2, the corollary is the special case where A1 = A, B1 = I, A2 = −I, and B2 = B. Thus condition a) of Theorem 2 is automatically satisfied. Moreover, B1∗ and A2 are clearly onto. Therefore b) reduces to the condition that at least one of A and B ∗ be onto, which is the conclusion of the corollary. In [1] there aresome partial results obtained on mappings of the form n (X1 , X2 , . . . , Xn ) → i=1 Ai Xi Bi where n ≥ 3. It is shown that the question of when Φ is onto can be reduced to the case where the coefficients {Ai } and {Bi } are projections. A necessary condition that a Φ with projection coefficients be onto is obtained. There is a sufficient condition established in the special case where each of the {Ai } and the {Bi } consists of commuting families of projections. Moreover, in the case n = 2, Theorem 2 above is also proven for Φ restricted to the algebra of compact operators and to the Schatten p-classes. However, necessary and sufficient conditions that such a mapping be onto are only given in the case n = 2. Thus there is more work to be done on such mappings. Another question left open in [1] is whether the corollary holds for operators on Banach spaces. More generally, the paper [1] does not contain any results on Banach spaces that are not Hilbert spaces.

44

P. Rosenthal

References [1] D. Hadwin, E. Nordgren, and P. Rosenthal, On the operator equation AXB+CY D = Z, Oper. Matrices, 1 (2007), no. 2, 199–207. [2] R. Bhatia and P. Rosenthal, How and Why to Solve the Operator Equation AX − XB = Y , Bull. London Math. Soc. 29 (1997), 1–21. [3] J. Dixmier, Etude sur les variétés et les opérateurs de Julia, avec quelques applications, Bull. Soc. Math. France 77 (1949), 11–101. [4] P.A. Fillmore, J.G. Stampfli, and J.P Williams, On the essential numerical range, the essential spectrum, and a problem of Halmos, Acta Sci. Math. (Szeged) 33 (1972), 179–192. [5] E. Nordgren and P. Rosenthal, Boundary Values of Berezin Symbols, Operator Theory: Advances and Applications, Vol. 73, Birkh¨ auser Verlag Basel/Switzerland, 1994. Peter Rosenthal Department of Mathematics University of Toronto Toronto, Ontario M5S 3G3 Canada e-mail: [email protected]

Topological Radicals, II. Applications to Spectral Theory of Multiplication Operators V.S. Shulman and Yu.V. Turovskii To Wojciech Wojty´ nski with gratitude for his wonderful results and stimulating questions.

Abstract. We develop the tensor spectral radius technique and the theory of the tensor Jacobson radical. Based on them we obtain several results on spectra of multiplication operators on Banach bimodules and indicate some applications to the spectral theory of elementary and multiplication operators on Banach algebras and modules with various compactness properties. Mathematics Subject Classification (2000). Primary 46H15, 46H25; Secondary 16Nxx. Keywords. Tensor spectral radius, joint spectral radius, tensor Jacobson radical, hypocompact radical, hypofinite radical, elementary operator, multiplication operator, semicompact operator, semifinite operator, hypocompact algebra, hypofinite algebra.

1. Introduction The localization of spectrum of an elementary operator in terms of spectra of its coefficients is one of the most popular subjects in the theory of elementary operators. The strongest results in this area were obtained for operators with commutative coefficient families because this allows one to use the theory of joint spectra (see [11]). Here we consider the less restrictive conditions than ncommutativity. For instance it is not known to us whether an operator T x = k=1 ak xbk on a Banach algebra A is quasinilpotent if a1 , . . . , an belong to a radical closed subalgebra of A. However, if all ai are compact operators, the answer is positive (see for example This paper is in final form and no version of it will be submitted for publication elsewhere.


45

46

V.S. Shulman and Yu.V. Turovskii

[28, Lemma 5.10]) and may be obtained by using the joint spectral radius technique. We consider multiplication operators of more general type than elementary ones as well as more general classes of coefficient algebras than algebras of compact operators. As a main technical tool we present the theory of tensor spectral radius initiated in [29] in the framework of the general theory of topological radicals. Based on it we obtain several results on spectra of multiplication operators on Banach bimodules and indicate their applications to spectral theory of elementary operators on Banach algebras with various compactness properties. Recall that an element a of a normed algebra A is called compact if the elementary operator x −→ axa on A is compact. The reason for such a definition is a well-known theorem of Vala [32] which states that a bounded operator on a Banach space X is compact iff it is a compact element of the algebra B(X) of all bounded operators on X. If all elements a ∈ A are compact then A is called compact. If, more strongly, for all a, b ∈ A, the operator x −→ axb on A is compact then A is called bicompact. A less restrictive condition is that A is generated as a normed algebra by the semigroup of all its compact elements. The most wide class of algebras of this kind is the class of hypocompact algebras. A normed algebra A is called hypocompact if each non-zero quotient of A by a closed ideal has a non-zero compact element. One may realize a hypocompact algebra as a result of a transfinite sequence of extensions of bicompact algebras. This class has some resemblance with the class of GCR-algebras in the C ∗ -algebras. Note for example that the image of each strictly irreducible representation of a hypocompact Banach algebra contains a non-zero finite rank operator. We show that elementary operators on hypocompact Banach algebras commutative modulo the Jacobson radical are spectrally computable, that is σ(T + S) ⊂ σ(T ) + σ(S) and σ(T S) ⊂ σ(T )σ(S) for all elementary operators T, S. Moreover, if all operators La −Ra are quasinilpotent on A (we call such algebras Engel ) then the spectra of elementary operators satisfy the inclusion Lak Rbk ⊂ σ ak bk . σ k

k

Among other applications we mention results on the structure of closed ideals in a radical compact Banach algebra. We prove that if such an algebra is infinite dimensional then it has infinite chains of ideals. As a consequence, we get that there is an infinite chain of closed operator ideals in the sense of Pietsch [20] intermediate between the ideals of approximable and compact operators. In the last section we consider the applications of the theory to spectral subspaces of multiplication operators. In 1978 Wojty´ nski, working on the problem of the existence of a closed two-sided ideal in a radical Banach algebra, proved the following result on linear operator equations with compact coefficients.

Topological Radicals, II. Applications

47

Lemma 1.1. [36] Let all coefficients a, b, ai , bi of the linear operator equation ax + xb +

n

ai xbi = λx

(1.1)

i=1

be compact operators on a Banach space X. If λ = 0 then each bounded solution x of (1.1) is a nuclear operator. The presence of nuclear operators gives a possibility to use trace for proving the quasinilpotence of some multiplication operators. Using this, Wojty´ nski proved in [36] that every radical Banach algebra having non-zero compact elements is not topologically simple (if dimension of the algebra is larger than 1). In [35] he applied the same argument to Banach Lie algebras and proved that if all adjoint operators of a Banach Lie algebra L are compact and quasinilpotent, then L has a non-trivial closed Lie ideal. Both results are now obtained in a more general setting with using another technique [31, 28], but Wojty´ nski’s approach itself is interesting and still helpful. Several years after [36], Fong and Radjavi [14] considered a more general class of equations (1.2) ai xbi = λx, where the sum is finite and for each i at least one of operators ai , bi is compact. They worked only in the case of Hilbert space operators but proved much more, namely that all solutions of (1.2) belong to each Schatten class Cp , p > 0. On the other hand, they showed that solutions of (1.2) are not necessarily finite rank operators: each operator x whose singular numbers decrease more quickly than every geometric progression is a solution of an equation of the form (1.2). We will show here that the main results of Wojty´ nski and of Fong and Radjavi extend to multiplication operators with infinite number of summands. Furthermore, we will see that not only eigenspaces with non-zero eigenvalues consist of nuclear operators but that the same holds for spectral subspaces corresponding to components of spectra non-containing 0 (or stronger, for all invariant subspaces on which the operator is surjective). Moreover, the ideal of nuclear operator here can be changed by any quasi-Banach ideal in the case of elementary operators (i.e., when the number of summands is finite). We prove also that the results extend to “integral multiplication operators”. In particular, if an operator x satisfies the condition

β

(a(t)xu(t) + v(t)xb(t))dμ = λx

with

λ = 0,

α

where a(t) and b(t) are continuous operator-valued functions, u(t) and v(t) are continuous compact operator-valued functions, then x is nuclear.

48


We also extend the results to systems of equations. A simple example is the following: Let x1 , . . . , xn satisfy a system of equations n

aik xk bik = λxi , i = 1, . . . , n, λ = 0.

k=1

If for each pair (i, k) at least one of operators aik , bik is compact then all xi are nuclear (moreover belong to each quasi-Banach operator ideal of B(X)). Such systems of equations arise, for example, in the study of subgraded Lie algebras [16]. Apart of tensor radical technique our approach is based on a general result (Theorem 6.5) which is not restricted by multiplication operators but deals with bounded operators on an ordered pair of Banach spaces. We would like to express our heartfelt gratitude to Niels Grønbæk for a very helpful discussion of the results of the paper [34] and to the referee for his patient and attentive reading of the manuscript and for numerous useful suggestions.

2. Preliminary results 2.1. Notation All spaces are assumed to be complex. If a normed algebra A is not unital, denote by A1 the normed algebra obtained by adjoining the identity element to A, and if the completion of A. The term A is already unital, let A1 = A. We denote by A ideal always means a two-sided ideal. If A is a normed algebra and I is an ideal of A then the term A/I always denotes the quotient of A by the closure of I in the norm of A (even if I is supplied with its own norm). It is convenient to write a/I for a + I ∈ A/I. Also, by a quotient of a normed algebra A we always mean any quotient of A by a closed ideal. Let A be a normed algebra. A norm ·A on A is called an algebra (or submultiplicative) norm if abA ≤ aA bA for all a, b ∈ A. If A has another norm (or seminorm), say ·, we write (A, ·) to indicate that A is considered with respect to ·. The norm · is equivalent to ·A on A if there are constants s, t > 0 such that s · ≤ ·A ≤ t · on A. Assume now that A is unital. Then ·A is called unital if 1A = 1. It is well known that every algebra norm on A is equivalent to a unital one. 2.2. Quasinilpotents and the radical modulo an ideal An element a of a normed algebra A is called quasinilpotent if 1/n

inf an n

= 0.

Let Q (A) denote the set of all quasinilpotent elements of A.


49

Let A be a normed algebra, and let distA (a, E), or simply dist (a, E), denote the distance from a ∈ A to E ⊂ A, that is dist (a, E) = inf {a − b : b ∈ E} . Let QE (A) be the set of all elements a ∈ A such that 1/n

inf distA (an , E) n

= 0.

If E = J is an ideal of A, then distA (a, J) is simply a quotient norm of q (a) in the quotient algebra A/J, where J denotes the closure of J in A and q : A −→ A/J is the standard quotient map. In other words, QJ (A) is the set of all a ∈ A quasinilpotent modulo J. Let rad (A) denote the Jacobson radical of A, and let radJ (A) denote the Jacobson radical of A modulo J, that is the preimage in A of the radical of the quotient algebra A/J. If A is complete, write Rad (A) instead of rad (A). Proposition 2.1. Let A be a Banach algebra and J be an ideal of A. Then (i) QJ (A) contains the intersection of all primitive ideals of A containing J (= RadJ (A)). (ii) If a closed subalgebra B ⊂ A is such that J ⊂ B and B/J is radical then B ⊂ QJ (A). (iii) An element a ∈ A belongs to QJ (A) if and only if for each ε > 0 there is n ∈ N such that dist(am , J) < εm for all m ≥ n. Proof. (i) As is known the Jacobson radical Rad(A) of a Banach algebra A is the largest ideal consisting of quasinilpotents. Hence QJ (A) contains the Jacobson radical of A modulo J. The last, as well known, is the intersection of all primitive ideals containing J. (ii) Straightforward. (iii) Follows immediately from the equality dist(a, J) = a/J. An algebra is usually said to be radical if it is Jacobson radical. The following lemma slightly improves the respective classical result. Lemma 2.2. Let A be a Banach algebra, and let I be an ideal of A. If I is radical and A/I is radical then A is radical. Proof. Let π be a strictly irreducible representation of A on X. We may assume that X is a Banach space and that π is continuous. If the restriction of π to I is non-zero then it is a strictly irreducible representation of I, in contradiction with the assumption that I is radical. Therefore π|I = 0 and, by continuity, I ⊂ ker π. Hence π defines a strictly irreducible representation of A/I. Since A/I is radical, this means that dim X = 1 and π = 0.

50


2.3. Normed subalgebras and flexible ideals 2.3.1. Spectrum with respect to a Banach subalgebra. Let A, B be normed algebras with norms ·A and ·B respectively, and let B be a subalgebra of A. We say that B is a normed subalgebra if ·A ≤ ·B on B. Every complete (with respect to ·B ) normed subalgebra B is called a Banach subalgebra. Let σA (a), or simply σ (a), denote the spectrum of a ∈ A with respect to A1 . Recall that this definition of spectrum coincides with Definition 5.1 in [9] in virtue of [9, Lemma 5.2]. Let σ A (a) denote the polynomially convex hull of σA (a), and 1/n let ρA (a), or simply ρ (a), denote the spectral radius of a defined as inf n an A . If A is a Banach algebra then ρA (a) = sup {|λ| : λ ∈ σA (a)} (Gelfand’s formula), and σ A (a) is received from σA (a) by filling the holes of σA (a). The term “clopen” means “closed and open simultaneously”. Proposition 2.3. Let A be a unital Banach algebra, and let B be a unital Banach subalgebra of A (i.e., the units for A and B coincide). Then (i) σA (a) ⊂ σB (a) for every a ∈ B, and each clopen subset of σB (a) has a non-void intersection with the polynomial hull σ A (a) of σA (a). (ii) If σB (A) is finite or countable then σA (a) = σB (a). Proof. It is evident that σA (a) ⊂ σB (a). To prove the second statement, suppose that σ1 is a clopen subset of σB (a) which doesn’t intersect σ A (a). Let p be the corresponding Riesz projection in B, 1 p= (λ − a)−1 dλ, 2πi Γ A (a). Then p = 0. where Γ surrounds σ1 and doesn’t intersect σ On the other hand, p = 0 because it can be regarded as a Riesz projection of a in A and there are no points of σA (a) inside Γ. The obtained contradiction proves (i). To show (ii), note that if σB (a) is countable then σA (a) is countable hence σ A (a) = σA (a). Thus each clopen subset of σB (a) intersects σA (a). Any point λ ∈ σB (a) is clearly the intersection of a sequence of clopen subsets of σB (a). Since all of them intersects σA (a) we get that λ ∈ σA (a). Thus σB (a) ⊂ σA (a) and we are done. When the subalgebra B is closed in A, the result is related to [22, Theorem 10.18]. The situation is especially simple if B is a (non-necessarily closed) ideal of A. Remark 2.4. Let I be an ideal of an algebra A and a ∈ I. It is easy to check that if (a − λ) b = 1 or b (a − λ) = 1 for b ∈ A1 and λ = 0, then b + λ−1 ∈ I. Hence, in virtue of [9, Lemma 5.2], {0} ∪ σI (a) = {0} ∪ σA (a).


51

2.3.2. Flexible ideals. Let A, I be normed algebras with norms ·A and ·I respectively, and let I be an ideal of A such that xA ≤ xI and axbI ≤ aA xI bA for all x ∈ I and a, b ∈ A1 . Such an algebra norm ·I on I is called flexible (with respect to ·A or (A, ·A ), naturally). An ideal having a flexible norm is called a flexible ideal. Every ideal I of a normed algebra A with ·I = ·A on I is of course flexible. By definition, a Banach ideal I of a normed algebra A is an ideal which is a Banach subalgebra of A. Lemma 2.5. Every Banach ideal of a Banach algebra is flexible with respect to an equivalent algebra norm. Proof. Let I be a Banach ideal of a Banach algebra A. By [4, Theorem 2.3], there is s > 0 such that axbI ≤ s aA xI bA for all x ∈ I and a, b ∈ A1 . Define ·I on I by xI = sup axbI : aA , bA ≤ 1, a, b ∈ A1 for every x ∈ I. It is easy to check that ·I is an algebra norm on I, ·I ≤ ·I ≤ s ·I . and

axbI ≤ aA xI bA for every a, b ∈ A1 . As ·A ≤ ·I on I, we obtain that I is a flexible ideal with respect to ·I . 2.3.3. Completion of normed subalgebras and ideals. If B is a normed subalgebra in a normed algebra A then the “identity” homomorphism i : (B, · B ) → A is → A. continuous and therefore extends by continuity to the homomorphism ˆı : B The proof of the following result is straightforward and we omit it. Lemma 2.6. Let B be a normed subalgebra of a normed algebra A. Then of the completion B of B in the completion A of A is a (i) The image ˆı(B) kerˆı. Banach subalgebra of A with respect to the norm ·∗ of the quotient B/ (ii) If B is a flexible ideal of A then ˆı(B) is a Banach ideal of A and its norm ·∗ is flexible. (A) or, simply, When it cannot lead to a misunderstanding, we will write B (·) B instead of ˆı(B).

52


2.3.4. Sums and intersections of Banach ideals. The following extends the class of examples of flexible ideals. Proposition 2.7. Let I and J be flexible ideals of a normed algebra A. Then (i) I ∩J is a flexible ideal of A with respect to the norm ·I∩J = max{·I ,·J }. (ii) I + J is a flexible ideal of A with respect to the norm zI+J = inf {xI + yJ : z = x + y, x ∈ I, y ∈ J} for every z ∈ I + J. (iii) I ∩ J is a flexible ideal of I + J. (iv) If I and J are Banach ideals then I ∩ J and I + J are Banach ideals with flexible norms ·I∩J and ·I+J respectively. Proof. It follows from [6, Lemma 2.3.1] that ·I∩J and ·I+J are norms and (iv) holds if (i) and (ii) hold. So it suffices to show that ·I∩J and ·I+J are flexible. It is easy to see that ·I∩J is flexible and ·A ≤ ·I+J on I + J. For z = x + y and z = x + y with x, x ∈ I and y, y ∈ J, we obtain that zz I+J ≤ xx + xy I + yx + yy J ≤ xx I + xy I + yx J + yy J ≤ xI x I + xI y A + yJ x A + yJ y J ≤ xI x I + xI y J + yJ x I + yJ y J ≤ (xI + yJ ) (x I + y J ) and azbI+J ≤ axbI + aybJ ≤ aA xI bA + aA yJ bA = aA (xI + yJ ) bA for every a, b ∈ A1 . Hence ·I+J is clearly a flexible norm. (iii) It is clear that I ∩ J is an ideal of I + J, ·I+J ≤ ·I∩J on I ∩ J, and flexibility of ·I∩J with respect to I + J follows from one with respect to A. In the situation of Proposition 2.7 it is convenient to call I ∩ J and I + J with their flexible norms a flexible intersection and a flexible sum of ideals I and J, respectively. An important class of examples of flexible ideals may be obtained by using the notion of normed operator ideals [20, 12]. Note that normed operator ideals in [20] are the same as Banach operator ideals in [12], and we prefer the terminology in [12]. Example 2.8. Let I be a Banach operator ideal. Then I (X) is a Banach ideal of B (X) for every Banach space X and its norm is flexible.


53

2.4. Projective tensor products 2.4.1. Tensor products of normed algebras. Let A1 ⊗ A2 denote the algebraic tensor product of normed algebras A1 and A2 , and let A1 ⊗γ A2 denote (A1 ⊗ A2 , γ), where γ is the projective crossnorm. Recall that γ is defined by ! γ (c) = inf ai ⊗ b i = c ai A1 bi A2 : for every c ∈ A1 ⊗γ A2 . Then A = A1 ⊗γ A2 is a normed algebra and γ is its algebra norm. To underline that the projective norm γ is considered in A1 ⊗ A2 , we write γ = γA1 ,A2 or γ = γA . We also write γ = γ · A , · A to indicate which 1 2 norm are considered in Ai . γ A2 , or simply A1 ⊗A 2 , we denote Let A1 and A2 be normed algebras. By A1 ⊗ the projective tensor product of A1 and A2 that is the completion of A1 ⊗γ A2 . By definition, it is a Banach algebra, and clearly it coincides with the projective 2 can be written tensor product of the completions of Ai . The elements of A1 ⊗A in the form ∞ ak ⊗ bk with ak bk < ∞, (2.1) c= k=1

k

2 is given by where ak ∈ A1 , bk ∈ A2 . Moreover, the norm · = γ (·) in A1 ⊗A ak bk , c = inf k

where inf is taken over representations of c in form (2.1). 2.4.2. Tensor products of normed subalgebras and ideals. If Bi is a subalgebra of an algebra Ai for i = 1, 2, then B1 ⊗γ B2 is a subalgebra of A1 ⊗γ A2 (see [10, Section 3.3.1]). If Ii is an ideal of Ai for i = 1, 2, then I1 ⊗γ I2 is clearly an ideal of A1 ⊗γ A2 . Proposition 2.9. Let A1 and A2 be normed algebras, and A = A1 ⊗γ A2 . Then (i) If Bi is a normed subalgebra of Ai for i = 1, 2, then B := B1 ⊗γ B2 with γB = γ · B , · B is a normed subalgebra of A. 1 2 (ii) If Ii is a flexible ideal of Ai for i = 1, 2, then I := I1 ⊗γ I2 with γI = γ · I , · I 1 2 is a flexible ideal of A and γI (azb) ≤ γA (a) γI (z) γA (b) for every a, b ∈ A1 and z ∈ I. Proof. (i) Indeed, the norm γB on B majorizes γA (and the equality does not hold in general even if · Bi = · Ai on Bi ). (ii) Straightforward. The natural embedding i of B1 ⊗γ B2 into A1 ⊗γ A2 extends by continuity to γ B2 into A = A1 ⊗ γ A2 . Let ˆı(B1 ⊗ γ B2 ) be a continuous homomorphism ˆı of B1 ⊗ γ B2 / kerˆı. We denote supplied with the norm inherited from the quotient B1 ⊗ (A) B2 or simply B1 ⊗ (·) B2 . this subalgebra by B1 ⊗

54


Taking into account Lemma 2.6 and Proposition 2.9, we obtain the following result. γ A2 . Then Corollary 2.10. Let A1 and A2 be normed algebras, and A = A1 ⊗ (·) B2 is a (i) If Bi is a normed subalgebra of Ai for i = 1, 2, then B := B1 ⊗ Banach subalgebra of A. (·) I2 is a Banach ideal (ii) If Ii is a flexible ideal of Ai for i = 1, 2, then I := I1 ⊗ of A and its norm (inherited from the respective quotient) is flexible. 2.4.3. Quotients of tensor products. 2 . Let Ji be Proposition 2.11. Let A1 and A2 be normed algebras and A = A1 ⊗A ideals of Ai for i = 1, 2, and let J = J1 ⊗ A2 + A1 ⊗ J2 . Then (i) The closure of J in A is an ideal of A, and A/J is topologically isomorphic to B = A1 /J1 ⊗ A2 /J2 . (·) A2 + A1 ⊗ (·) I2 is (ii) If Ii are closed ideals of Ai containing Ji , and if I := I1 ⊗ a flexible sum of Banach ideals in A, then the closure of J in I is an ideal of I and I/J is topologically isomorphic to the algebra (·) (·) A2 /J2 + A1 /J1 ⊗ I2 /J2 Q = I1 /J1 ⊗ 2 /J2 ). taken with the norm of the flexible sum of Banach ideals in (A1 /J1 )⊗(A Proof. (i) The first statement follows from the fact that J is an ideal of the algebraic tensor product A1 ⊗ A2 . To show the second one, assume first that A1 and A2 are Banach algebras, and that Ji is a closed ideal of Ai for i = 1, 2. As usual, we denote by qJi the standard epimorphisms from Ai to Ai /Ji and 2 to A/J. Setting by qJ the standard epimorphism from A = A1 ⊗A φ((a1 + J1 )⊗(a2 + J2 )) = qJ (a1 ⊗a2 ), 2 /J2 ) → A/J such that the we obtain a bounded homomorphism φ : (A1 /J1 )⊗(A diagram A/J gN > NNN ~ NNNφ ~ qJ ~ NNN ~ ~ NN ~ ~~ qJ1 ⊗qJ2 / (A1 /J1 )⊗(A 2 /J2 ) A is commutative. Since qJ is surjective, φ is surjective. On the other hand, it is easy to see that 2 /J2 ) then z = qJ1 ⊗ qJ2 is surjective. So, if φ(z) = 0 for some z ∈ (A1 /J1 )⊗(A (qJ1 ⊗ qJ2 ) (a) for some a ∈ A. In fact, we have that a ∈ J by the commutativity of the diagram. But J ⊂ ker(qJ1 ⊗qJ2 ), whence z = 0. This implies that φ is injective. Thus φ establishes a bounded isomorphism of B and A/J. By the Banach Theorem, this isomorphism is topological.


55

In the general case, passing to completions of Ai and to closures of Ji and applying Proposition 2.6 and simple identifications in completions of quotients of normed algebras, we get the result. (ii) Follows from a similar analysis of the commutative diagram I/J ÀA ~? AAψ ~ ~ AA ~~ AA ~ ~ ~ q /Q I qJ

where q is the map sending p1 ⊗a2 + a1 ⊗p2 to (p1 /J1 )⊗(a2 /J2 ) + (a1 /J1 )⊗(p2 /J2 ) for all pi ∈ Ii , ai ∈ Ai (i = 1, 2). The existence of ψ is evident, surjectivity of q can be verified in a standard way.

3. Tensor Jacobson radical 3.1. Tensor spectral radius of a summable family Let A be a normed algebra. We will call by families arbitrary sequences of elements ∞ of A; two families are equivalent (write {an }∞ 1 {bn }1 ) if one of them can be obtained from the other by renumbering. The equivalence classes can be considered as countable generalized subsets [29]: to characterize the class determined by a sequence one have only to indicate which elements of A come into the sequence and how many times. By definition [29, Section 3.4], a generalized subset S of A is a cardinal-valued function κS defined on A. The set {a ∈ A : κS (a) > 0} is called a support of S. One can regard usual subsets N ⊂ A as generalized ones, identifying the indicator κN of N with N . A generalized subset S of A is countable if its support and κS (a) are (finite or) countable for every a from the support of S. Let S and P be generalized subsets of A. The inclusion S ⊂ P means κS (a) ≤ κP (a) for every a ∈ A. We define the disjoint union S P of generalized subsets of A by κSP (a) = κS (a) + κP (a) for every a ∈ A. Disjoint union of a collection of generalized subsets is defined similarly. In particular, for an integer n > 0, the disjoint union of n copies of S will be denoted by n • S. We define the product SP of generalized subsets of A by κS (b)κP (c) κSP (a) = (b,c)∈A×A, bc=a

for every a ∈ A.

56

V.S. Shulman and Yu.V. Turovskii Given a generalized subset S of A, put κS (a)a η(S) = a∈A

and S =

sup {a : a ∈ A, κS (a) > 0} .

κS (a)>0

If η(S) < ∞ then S is called summable, and if S < ∞ then S is called bounded. To each sequence M = {an }∞ n=1 in A there corresponds a countable generalized subset S = S(M) by the rule κS (a) = card{n : an = a} for every a ∈ A. We say that M is a representative of S. In terms of representa∞ tives M = {an }∞ 1 and N = {bn }1 the family M N corresponds to the two-index ∞ sequence {an bm }n,m=1 which can be renumbered in an arbitrary way, while M N corresponds to the sequence {cn }∞ 1 with c2k−1 = ak , c2k = bk . It is obvious in this context that ∞ M N ∞ i=1 ai N j=1 M bj , ∞

∞

where aN = {abn }1 and M b = {an b}1 as usual. In particular, M (N1 N2 ) M N1 M N2 and (M1 M2 ) N M1 N M2 N for any families Mi and Ni , i = 1, 2, and (M N ) K M (N K) for any families in A. Set M M , M M n

1

M

n+m

n−1

M M n

(3.1)

M for every n > 0. By (3.1)

m

(3.2)

for every n, m ∈ N. For two families M and N in A, we say that M is a subfamily of N (write M N ) if S(M) ⊂ S(N ) for corresponding generalized subsets of A. Now let S be a summable generalized subset of A. This is equivalent to the condition that S has a representative M in 1 (A), i.e., S = S(M) for some M ∈ 1 (A). Using this and setting η(M ) = η S(M) , we simply write “a family M = {an }∞ 1 in A is summable”. Moreover, M 1 (A) = η(M ) = η(N ) for every N M . Let M and N be summable families in A. It is evident that η(M N ) = η(M ) + η(N ) and η(M N ) η(M )η(N ). We obtain by (3.2) and (3.3) that η(M n+m ) η(M n )η(M m )

(3.3) (3.4)


57

for every n, m ∈ N. It follows from (3.4) that, for every summable family M , there exists a limit ρt (M ) = lim(η (M n ))1/n = inf(η(M n ))1/n . The number ρt (M ) is called a tensor spectral radius of M . As (M m )n M mn for every n, m ∈ N, then ρt (M m )1/m = (lim(η((M m )n ))1/n )1/m = lim(η(M mn ))1/nm = ρt (M ). n

n

(3.5)

Now let B be a normed algebra, and let S be a bounded countable generalized subset of B. Then S has L in ∞ (B), i.e., S = S(L) for some L ∈ a representative ∞ (B). Setting L = S(L) , we write “a family L = {bn }∞ 1 in B is bounded”. Moreover, L∞ (B) = L = K for every K L. A usual countable subset N of B is bounded if and only if supb∈N b < ∞. Let L and K be bounded families in K. It is evident that L K = max {L , K} and LK ≤ L K . It follows as above that, for every bounded family L, there is a limit ρ(L) = lim(Ln )1/n = inf(Ln )1/n . The number ρ(L) is called a joint spectral radius of L. It is clear that ρ(Lm )1/m = ρ(L) for every m ∈ N. ∞ ∞ Let A and B be normed algebras, M = {an }1 ∈ 1 (A) and L = {bn }1 ∈ ∞ which is equal to n=1 an ⊗ bn. It is ∞ (B). Let M⊗ L denote an element of A⊗B there are M ∈ 1 (A) clear (see also Section 2.4.1) that for every element z ∈ A⊗B and L ∈ ∞ (B) such that z = M⊗ L. The following theorem justifies the term “tensor spectral radius”. Theorem 3.1. Let A be a normed algebra. Then (i) ρ (M⊗ L) ≤ ρt (M )ρ(L) for every normed algebra B, M ∈ 1 (A) and L ∈ ∞ (B). (ii) There are a unital Banach algebra B, L ∈ ∞ (B), and a bounded linear such that M⊗ L = η(M ) operator T : M −→ M⊗ L from 1 (A) into A⊗B and ρ(M⊗ L) = ρt (M ) for every M ∈ 1 (A). ∞

∞

Proof. (i) Let L = {bn }1 ∈ ∞ (B). Then for every M = {an }1 ∈ 1 (A) we have that k k an1 · · · ank bn1 · · · bnk ≤ η(M k ) Lk (M⊗ L) = γ (M⊗ L) ≤ n1 ,...,nk

Taking k-roots and passing to limits, we obtain that ρ (M⊗ L) ≤ ρt (M )ρ(L). (ii) Let G be the free unital semigroup with a countable set W = {wk }k1 of generators. That is G = ∪m0 Wm , where W0 = 1, Wm is the direct product of m copies of W realized as the set of ‘words’ wk1 wk2 . . . wkm of the length m, and the multiplication is lexical. Let B = 1 (G) be the corresponding semigroup algebra.

58

V.S. Shulman and Yu.V. Turovskii ∞ Let L = {wn }∞ 1 . For any M = {an }1 ∈ 1 (A), we have that

M⊗ L =

∞

ak ⊗wk .

k=1

and Then T : M −→ M⊗ L is a bounded linear operator from 1 (A) into A⊗B n T (M ) = ak1 . . . akn ⊗wk1 . . . wkn . k1 ,...,kn

1 (G) is isometrically isomorphic via the map defined by (a⊗f )(g) −→ Since A⊗ f (g)a to the Banach algebra 1 (G, A) of all summable A-valued functions on G, then n ak1 . . . akn = η(M n ). T (M ) = k1 ,...,kn

It follows that

ρ(M⊗ L) = ρt (M ).

We write η · (M ) instead of η(M ) if there is a necessity to indicate which norm in A is meant. Proposition 3.2. Let M be a summable family in a normed algebra A. Then ρt (M ) doesn’t change if the norm on A is changed by an equivalent norm.

1/m

Proof. If · ≤ t · for some t > 0, then lim η · (M m ) ≤ lim η · (M m ) so that the opposite inequality for norms implies the equality of limits.

1/m

,

∞ For summable families M = {a n } and N = {bn }1 , let M ∗ N = {cn } denote the convolution of M and N : cn = i+j=n+1 ai bj for every n > 0. ∞

∞

Proposition 3.3. If M = {an }1 and N = {bn }1 are summable families in A then ρt (M ∗ N ) ≤ ρt (M N ) = ρt (N M ) and ρt (M + N ) ≤ ρt (M N ). Proof. Note that η (M N )n+1 ≤ η (M ) η ((N M )n ) η (N ) for every n. This implies that ρt (M N ) ≤ ρt (N M ). Changing M and N by places, we have the equality. We have that ⎛ ⎞ ⎛ ⎞ k ⎝ η (M ∗ N ) = ai1 bj1 ⎠ · · · ⎝ aik bjk ⎠ n1 ,...,nk ii +ji =n1 +1 ik +jk =nk +1 ≤ ··· ai1 bj1 · · · aik bjk n1 ,...,nk ii +ji =n1 +1

=

ik +jk =nk +1

an1 bn2 · · · an = η (M N )k b n 2k−1 2k

n1 ,...,n2k

for every k > 0, whence ρt (M ∗ N ) ≤ ρt (M N ).


59

Further, M + N = {an + bn }∞ 1 and ∞

k

(M + N ) {(an1 + bn1 ) · · · (ank + bnk )}n1 ,...,nk =1

= {an1 · · · ank + bn1 an2 · · · ank + · · · + bn1 · · · bnk }∞ n1 ,...,nk =1 .

Then k η (M + N ) =

an1 · · · ank + bn1 an2 · · · ank + · · · + bn1 · · · bnk

n1 ,...,nk

≤

(an1 · · · ank + bn1 an2 · · · ank + · · · + bn1 · · · bnk )

n1 ,...,nk

=

an1 · · · ank +

n1 ,...,nk

···+

bn1 an2 · · · ank + · · ·

n1 ,...,nk

bn1 · · · bnk

n1 ,...,nk

= η M k + η N M k−1 + · · · + η N k k = η M k N M k−1 · · · N k = η (M N ) for every k > 0, whence ρt (M + N ) ≤ ρt (M N ).

∞

As A is embedded into A1 , let M 0 be {xn }1 with xn = 0 for every n > 1 0 0 0 0 0 and x1 = 1, the identity element of A1 . Note 0 that M M M N , M N 0 N M N for any family N in A, and η M = 1. We say that families M and N in A commute if M N N M . This of course doesn’t mean that elements of M commute with elements of N . But the reverse statement clearly keeps: if each element of M commutes with each element of N then M and N commute. In particular, if N consists of elements of the center of A then M and N commute. Proposition 3.4. Let M and N be summable families in A. If M and N commute then ρt (M N ) ≤ ρt (M ) ρt (N ) and ρt (M N ) ≤ ρt (M ) + ρt (N ). n

n

Proof. Indeed, (N M ) N n M n and η ((N M ) ) = η (N n M n ) ≤ η (N n ) η (M n ) for every n. Taking n-roots and passing to limits, we obtain that ρt (M N ) ≤ ρt (M ) ρt (N ) . It is easy to see that

n (M N ) ni=0 Cni • M i N n−i

for every n > 0, whence η ((M N )n ) =

n i=0

n Cni η M i N n−i ≤ Cni η M i η N n−i . i=0

60


Let ε > 0, and take s ≥ 1 such that η M i ≤ s (ρt (M ) + ε)i and η N i ≤ i s (ρt (N ) + ε) for every i ∈ N. Then η ((M N )n ) ≤ s2

n

Cni (ρt (M ) + ε)i (ρt (N ) + ε)n−i

i=0 n

2

= s (ρt (M ) + ρt (N ) + 2ε)

for every n > 0. Taking n-roots and passing to limits, we obtain that ρt (M N ) ≤ ρt (M ) + ρt (N ) + 2ε As ε is arbitrary, we have that ρt (M N ) ≤ ρt (M ) + ρt (N ).

3.2. Absolutely convex hulls and tensor quasinilpotent families Let A be a normed algebra. A summable family M of elements of A is called tensor quasinilpotent if ρt (M ) = 0. The following result is an immediate consequence of Theorem 3.1(i). Corollary 3.5. If a family M in A is tensor quasinilpotent then for each bounded family L in a normed algebra B the element M⊗ L is quasinilpotent in A⊗B. ∞

}1 , let abst (M ) denote the set of all famFor a summable family M = {a n∞ ∞ ∞ ilies N = {bn }1 such that bm = nm an , where the sequences {tnm }m=1 n=1 t ∞ of complex numbers satisfy the condition m=1 |tnm | ≤ 1. We call abst (M ) the absolutely convex hull of M . To justify the term, note that abst (M ) is a closed absolutely convex subset of 1 (A). ∞

Proposition 3.6. If M = {an }1 is a summable family of elements of a normed ∞ algebra A then ρt (N ) ≤ ρt (M ) for any N = {bn }1 ∈ abst (M ). Proof. Indeed, k

η(N ) =

bm1 · · · bmk =

m1 ,...,mk

≤

m1 ,...,mk

n1 ,...,nk m1

≤

tn1 m1 · · · tnk mk an1 · · · ank n1 ,...,nk

|tn1 m1 · · · tnk mk | an1 · · · ank

m1 ,...,mk n1 ,...,nk

=

|tn1 m1 | · · ·

|tnk mk | an1 · · · ank

mk

an1 · · · ank = η(M k )

n1 ,...,nk

for every k, whence ρt (N ) ≤ ρt (M ).

Recall that a set K in a normed space X is called absolutely convex if t1 x1 + t2 x2 ∈ K for every integer n > 0 and for any x1 , x2 ∈ K and t1 , t2 ∈ C with |t1 |+|t2 | ≤ 1. If K is a compact set in X, then the number max {x − y : x, y ∈ K} is called the diameter of K and denoted by diam (K).


61

Lemma 3.7. Let X be a Banach space and let {Kn} be a sequence of absolutely convex compact sets in X such that diam (Kn ) < ∞. Then Kn is an absolutely convex compact set. Proof. It is clear that Kn is absolutely convex. To see that it is compact, note that the direct product K = Π∞ n=1 Kn is compact. Since 0 ∈ Kn , we get that a ≤ diam(Kn )/2 for a ∈ Kn . So there are numbers αn > 0 with a ≤ αn for a ∈ Kn , such that αn < ∞. It follows that the map ϕ : K −→ X defined by the formula ∞ ϕ({an }∞ ) = an , n=1 is continuous. As

n=1

Kn = ϕ(K), it is compact.

∞ {an }1

Let M = be a summable family in a Banach algebra A, and let Ω (M ) denote the set of all elements of the form tn an for complex numbers |tn | ≤ 1. Corollary 3.8. If M is a summable family in a Banach algebra A then Ω (M ) is an absolutely convex compact set such that ρ (a) ≤ ρt (M ) for any a ∈ Ω (M ). ∞ Moreover, Ω (M ) = { bn : {bn }1 ∈ abst (M )}. Proof. Indeed, Ω (M ) is the countable sum of absolutely convex compact sets {tan : |t| ≤ 1}, the sum of whose diameters is finite. So Ω (M ) is a convex compact set in A. Further, for any a = tn an ∈ Ω (M ), we have that k a ≤ tn1 · · · tnk an1 · · · ank ≤ an1 · · · ank = η M k n1 ,...,nk

n1 ,...,nk

for every k ∈ N. So ρ (a) ≤ ρt (M ). The last assertion easily follows from well-known properties of absolutely summable series. Corollary 3.9. Let M = {an }∞ 1 be a tensor quasinilpotent family in a Banach algebra A. Then every element of Ω (M ) is quasinilpotent. Lemma 3.10. If M = {an }∞ 1 is a summable family in A such that ρt (M ) < 1 then −1 ∞ m m the family m=1 M is summable and ρt (∞ . m=1 M ) = ρt (M ) (1 − ρt (M )) n for any k. Take t satisfying ρt (M ) < t < 1. Then Proof. Let M(k) ∞ n=k M there is an integer p such that η (M n ) ≤ tn for every n ≥ p. Then ∞

p−1

n

η (M )
0, whence ρt (M ) (1 − ρt (M ))

+

≤ ρt M(1) .

Corollary 3.11. If M = {an }∞ 1 is a tensor quasinilpotent family in a normed algebra A then the subalgebra generated by M consists of quasinilpotents. m Proof. Indeed, ρt (∞ m=1 M ) = 0 by Lemma 3.10, and every element of the subm algebra lies in ∪t>0 t Ω (∞ m=1 M ) which consists of quasinilpotents by Corollary 3.9.

3.3. Upper semicontinuity and subharmonicity of the tensor spectral radius Let A be a normed algebra. Since sequences in 1 (A) determine summable families, the tensor spectral radius can be considered as a function on 1 (A). We are going to show that this function is upper semicontinuous and subharmonic. ∞ If G is a subset of A, let F1 (G) be the set of all summable families M = {an }1 with all an ∈ G. In particular, F1 (A) = 1 (A). Clearly F1 (G) is a metric space with respect to the metric d(M, N ) = η(M − N ) induced by the norm on 1 (A). If A is a Banach algebra and G is a closed subset of A then F1 (G) is a complete metric space. Now we are able to establish the upper semicontinuity of the tensor spectral radius. Proposition 3.12. Let M ∈ 1 (A). For every ε > 0, there is δ > 0 such that ρt (N ) ≤ ρt (M ) + ε for every N ∈ 1 (A) satisfying d (N, M ) < δ. Proof. Let T : M −→ M⊗ L be the map defined in Theorem 3.1(ii). As the usual spectral radius is upper semicontinuous, to any ε > 0, there corresponds δ > 0 such that ρ (T (N )) ≤ ρ (T (M )) + ε for every N ∈ 1 (A) satisfying T (N ) − T (M ) < δ. By Theorem 3.1(ii), we get that ρ (T (N )) = ρt (N ), ρ (T (M )) = ρt (M ) and T (N ) − T (M ) = T (N − M ) = η(M − N ) = d(M, N ).


63

Let be an analytic function on an open set D ⊂ C with values in 1 (A). This means that for every λ ∈ D there is (λ) ∈ 1 (A) such that (μ) − (λ) μ→λ μ−λ in the norm of 1 (A). Clearly as an 1 (A)-valued function induces the family ∞ {fn}∞ 1 of A-valued functions on D: = {fn }1 . These functions are analytic since ∞ λ −→ (λ) = {fn (λ)}1 is analytic on D. So one can write = {fn }∞ 1 , etc. ∞ Let now = {fn }∞ and Ψ = {ψ } be (A)-valued functions on D. n 1 1 1 Let Ψ denote some sequence {ϕk }∞ of functions on D which is obtained from 1 {fnψm }∞ by renumbering. We will assume that the renumbering for this opn,m=1 eration is fixed, so Ψ is defined correctly. In such a case the functions ϕk are A-valued functions on D, and we write that Ψ = {ϕk } is an 1 (A)-valued function on D. (λ) = lim

∞

∞

Lemma 3.13. If = {fn }1 and Ψ = {ψn }1 are analytic 1 (A)-valued function on D then Ψ is an analytic 1 (A)-valued function on D. Proof. Indeed, the derivation (Ψ) exists and is clearly obtained from two ∞ index sequence {fn ψm + fn ψm }n,m=1 by the same renumbering as Ψ from ∞ {fnψm }n,m=1 . Moreover, η (Ψ) (λ) = fn (λ) ψm (λ) + fn (λ) ψm (λ) n,m

≤

(fn (λ) ψm (λ) + fn (λ) ψm (λ))

n,m

=

n

fn (λ)

ψm (λ) +

m

n

fn (λ)

ψm (λ)

m

= η ( (λ)) η (Ψ (λ)) + η ( (λ)) η (Ψ (λ)) < ∞.

1 m = m−1 For an 1 (A)-valued function = {fn }∞ 1 , let = and ∞ m for m > 1, so that = {φn }1 is an 1 (A)-valued function for some A-valued functions φn on D. It follows by induction from the definition of product of two functions that m (λ) (λ)m (3.6) for every λ ∈ D.

Corollary 3.14. If is an analytic 1 (A)-valued function on D then m is an analytic 1 (A)-valued function on D for every m > 0.

Proof. Follows from Lemma 3.13 by induction. m

For an 1 (A)-valued function , the function λ −→ η ( (λ) ) doesn’t depend on a renumbering. Moreover, it follows from (3.6) that m

η ( (λ) ) = η (m (λ)) for every λ ∈ D.

(3.7)

64


Lemma 3.15. If is an analytic 1 (A)-valued function on D then the functions m m λ −→ η ( (λ) ) and λ −→ log (η ( (λ) )) are subharmonic on D for all m. Proof. As η(·) is a norm on 1 (A), it is well known (see [33]) that λ −→ η ( (λ)) is a subharmonic function. Then it follows from Corollary 3.14 and (3.7) that λ −→ η ( (λ)m ) is subharmonic for every m. ∞ Let m = {φn }1 and β ∈ C. As m

∞

|exp (βλ)| η ( (λ) ) = η ({exp (βλ) φn (λ)}1 ) ∞

by (3.7) and λ −→ {exp (βλ) φn (λ)}1 determines an analytic 1 (A)-valued funcm tion on D, then, by above, λ −→ |exp (βλ)| η ( (λ) ) is subharmonic for every β ∈ C. It follows from Rado’s theorem [2, Appendix 2, Theorem 9] that the function λ −→ log (η ( (λ)m )) is subharmonic on D. We use Vesentini’s argument for subharmonicity of the usual spectral radius [33] in the following Theorem 3.16. If is an analytic 1 (A)-valued function on D then the functions log (ρt ()) : λ −→ log (ρt ( (λ))) and ρt () : λ −→ ρt ( (λ)) are subharmonic on D. 2 2m+1 2m Proof. As η (λ) ≤ η (λ) , the function λ −→ log (ρt ( (λ))) is ! 2m of a pointwise limit of the decreasing sequence λ −→ 2−m log η (λ) subharmonic functions and is therefore a subharmonic function by Theorem 1 of [2, Appendix 2]. Since the function t −→ exp (t) is convex and positive for t ∈ R, then the function λ −→ exp (log (ρt ( (λ)))) = ρt ( (λ)) is also subharmonic by the same theorem. 3.4. The ideal Rt (A) for a normed algebra A ∞ Let a be an element of a normed algebra A, and let M = {an }n=1 be a summable ∞ family in A. Let {a} M denote the family {xn }1 with x1 = a and xn = an−1 for n > 1. Note that the useful relation ({a} M ) ({b} N ) {ab} aN M b M N

(3.8)

is valid by our conventions, for any a, b ∈ A and M, N ∈ 1 (A). Let Rt (A) be the set of all a ∈ A such that ρt ({a} M ) = ρt (M ) for every M ∈ 1 (A). It is evident that Rt (A) consists of quasinilpotent elements of A. Lemma 3.17. Let a ∈ A. If there is s > 0 such that ρt ({a} M ) ≤ sρt (M ) for every M ∈ 1 (A) then a ∈ Rt (A). Proof. As the function μ −→ ρt ({μa} M ) is subharmonic by Theorem 3.16 and bounded on C, it is constant, whence ρt ({a} M ) = ρt (M ). 1 1 Lemma 3.18. Rt (A) = A ∩ Rt A . If A is complete then Rt (A) = Rt A .


65

Proof. Let A be non-unital and a ∈ Rt (A). As A1 = A ⊕ C, for every summable family M in A1 there are N ∈ 1 (A) and K ∈ 1 (C) such that M = N + K. Since N and K commute, then ρt (M ) = ρt (N + K) ≤ ρt (N K) ≤ ρt (N ) + ρt (K) by Propositions 3.3 and 3.4. Hence, as {μa} N and {0} K commute, ρt ({μa} M ) ≤ ρt ({μa} N + {0} K) ≤ ρt ({μa} N ) + ρt ({0} K) = ρt (N ) + ρt (K) < ∞ for every μ ∈ C. Therefore μ −→ ρt ({μa} M ) is constant, and as a consequence, ρt ({a} M ) = ρt (M ) . R (A) ⊂ Rt A1 . As M is arbitrary, a ∈ Rt A . So 1 t On the other hand, A ∩ Rt A ⊂ Rt (A) by definition. So we obtain that Rt (A) = A ∩ Rt A1 . Let now A be complete. We show that Rt A1 ⊂ A. Indeed, if a−λ ∈ Rt A1 with a ∈ A and λ ∈ C then a − λ is a quasinilpotent element of A1 . This means that the spectrum σ (a) of a is equal to {λ}. But, as a ∈ A and A is not unital, σ (a) contains zero. Thereforeλ = 0. So, if A is complete, Rt A1 ⊂ A, whence Rt A1 = Rt (A) by the above.

1

Theorem 3.19. Rt (A) is a closed ideal of A. Proof. Consider first the case when A has the identity element 1. Let a, b ∈ Rt (A). As ρt ({μa} M ) = |μ| ρt {a} μ−1 M = |μ| ρt μ−1 M = ρt (M ) for every M ∈ 1 (A) and non-zero μ ∈ C, then μa ∈ Rt (A) for every μ ∈ C. Since 2−1 (a + b) M ∈ abst ({a} {b} M ) for every M ∈ 1 (A), then, by Proposition 3.6, ρt 2−1 (a + b) M ≤ ρt ({a} {b} M ) = ρt (M ) , whence a + b ∈ Rt (A) by Lemma 3.17. So Rt (A) is a subspace of A. Let x ∈ A. Then ρt ({μa} {1} {x} M ) ≤ t0 := ρt ({1} {x} M ) < ∞ for every μ ∈ C. Therefore ρt ({μa} {1} {x} M )2 ≤ t20 2

by (3.5). Since {μax} M is a subfamily of ({μa} {1} {x} M ) in virtue of (3.8), we obtain that ρt ({μax} M ) ≤ t20 for every μ ∈ C and for every M ∈ 1 (A) with t0 depending only on x and M . Therefore μ −→ ρt ({μax} M ) is bounded on C. As this function is subharmonic,

66


it is constant, whence ax ∈ Rt (A), and, similarly, xa ∈ Rt (A). Thus Rt (A) is an ideal of A. Note that ρt ({a + x} M ) ≤ ρt ({2a} {2x} M ) = ρt ({2x} M ) ≤ η ({2x} M ) = 2x + η (M ) for every a ∈ Rt (A) and x ∈ A. Now if c is in the closure of Rt (A), then for every μ ∈ C there are a ∈ Rt (A) and x ∈ A with x ≤ 1 such that μc = a + x. Hence ρt ({μc} M ) = ρt ({a + x} M ) ≤ 2 + η (M ) for every M ∈ 1 (A). So μ −→ ρt ({μc} M ) is bounded and therefore constant, whence c ∈ Rt (A). Thus Rt (A) is a closed ideal of A. Now assume that A is not unital. We already have proved that Rt A1 is a closed ideal of A1 . Then Lemma 3.18 shows that Rt (A) is a closed ideal of A. Corollary 3.20. If A is a Banach algebra then Rt (A) ⊂ Rad (A). Proof. Indeed, Rt (A) is an ideal of A consisting of quasinilpotents. So we have that Rt (A) ⊂ Rad (A). Theorem 3.21. If ρt (aM ) = 0 for every M ∈ 1 (A) then a ∈ Rt (A). Proof. Let first A have the identity element 1. Let M ∈ 1 (A). Multiplying M by a scalar, one can assume that η (M ) < 1.

(3.9)

i 0 Then N := ∞ i=0 M is a summable family in A by Lemma 3.10, where M = ∞ {xn }n=1 with x1 = 1 and xi = 0 for every i > 1 as usual. By condition, we have that ρt (aN ) = 0. Let μ ∈ C be non-zero and take ε > 0 such that ε |μ| < 2−1 . Then there is t > 0 such that n

η ((aN ) ) ≤ tεn

(3.10)

for every n > 0. We have that n

η (({μa} M ) ) =

n i=0

(3.9)

|μ|

i

η (M m0 aM m1 · · · aM mi )

i

≤ η (M n ) +

k=0 mk =n−i n i

|μ|

i=1

i

η (aM m1 · · · aM mi )

k=0 mk =n−i

for every n > 0. As, for every i > 0, the number of summands η (aM m1 · · · aM mi ) is less than or equal to 2n and every such summand is less than or equal to


67

i η (aN ) , then we obtain that η (({μa} M )n ) ≤ η (M n ) + 2n

n

n (3.10) |μ|i η (aN )i ≤ η (M n ) + 2n t0 |μ|i εi

i=1

i=1

≤ η (M n ) + 2nt ≤ 2 max {η (M n ) , 2n t} Taking n-roots and passing to limits, we get ρt ({μa} M ) ≤ max {ρt (M ) , 2} for every μ ∈ C. As the function μ −→ ρt ({μa} M ) is bounded and subharmonic, it is constant. Therefore ρt ({a} M ) = ρt (M ) . As M is arbitrary, a ∈ Rt (A). Now assume that A is not unital. Then, for each M ∈ 1 A1 , the family K = M aM belongs to 1 (A), and ρt ((aM )2 ) = ρt (aK) = 0 by condition. By (3.5), we obtain that ρt (aM ) = 0 for every M ∈ 1 A1 , whence a ∈ Rt A1 by the proof above. Now the result follows from Lemma 3.18. 3.5. Tensor quasinilpotent algebras and ideals Let A be a normed algebra. A subset G of A is called a tensor quasinilpotent set if all summable families with elements in G are tensor quasinilpotent. A tensor quasinilpotent ideal in A is an ideal which is a tensor quasinilpotent subset of A. Theorem 3.22. ρt (M N ) = ρt (N ) for every M ∈ 1 (Rt (A)) and N ∈ 1 (A). ∞ Proof. Let N ∈ 1 (A), M = {an }∞ n=1 ∈ 1 (Rt (A)) and Mk = {an }n=k for every integer k > 0. For every ε > 0 and μ ∈ C, there is k > 0 such that η (μMk ) < ε. Then μ −→ μM N is an analytic function and

ρt (μM N ) = ρt (μM2 N ) = · · · = ρt (μMk N ) ≤ η (μMk N ) = η (μMk ) + η (N ) < η (N ) + ε. So μ −→ ρt (μM N ) is bounded and therefore constant. Hence we obtain that ρt (M N ) = ρt (N ). As a consequence, we obtain the following Corollary 3.23. Rt (A) is a tensor quasinilpotent ideal. Corollary 3.24. ρt (M + N ) = ρt (N ) and ρt (M ∗ N ) = ρt (M N ) = 0 for every M ∈ 1 (Rt (A)) and N ∈ 1 (A). Proof. Let μ ∈ C. Then ρt (μM + N ) ≤ ρt (μM N ) = ρt (N ) by Proposition 3.3 and Theorem 3.22. As μ −→ ρt (μM + N ) is subharmonic and bounded on C, it is constant, whence ρt (M + N ) = ρt (N ). Since M N ∈ 1 (Rt (A)), then ρt (M N ) = 0 by Corollary 3.23. Then we obtain that ρt (M ∗ N ) = 0 by Proposition 3.3. Corollary 3.25. Let A be a normed algebra and a ∈ A. The following conditions are equivalent:

68


(i) a ∈ Rt (A). (ii) ρt (aM ) = 0 for every M ∈ 1 (A). Proof. Indeed, (i) =⇒ (ii) follows from Corollary 3.24, and (ii) =⇒ (i) was proved in Theorem 3.21. We will prove now that Rt (A) is the largest tensor quasinilpotent ideal. Theorem 3.26. If I is a tensor quasinilpotent (possible, one-sided) ideal of A then I ⊂ Rt (A). Proof. Let I be a right ideal of A, and let a ∈ I. Then aM ∈ 1 (I) for every M ∈ 1 (A). As I is tensor quasinilpotent then ρt (aM ) = 0 for every M ∈ 1 (A). By Theorem 3.21, a ∈ Rt (A). So I ⊂ Rt (A). If I is a left ideal of A and a ∈ I, then ρt (aM ) = ρt (M a) by Proposition 3.3. So ρt (aM ) = 0 for every M ∈ 1 (A). We have again that a ∈ Rt (A). ∞

Lemma 3.27. Let M = {an }1 be a summable family in A and g : A −→ B ∞ be a bounded homomorphism of normed algebras. Then g (M ) := {g (an )}1 is a summable family of B and ρt (g (M )) ≤ ρt (M ). Proof. Indeed, it suffices to note that η (g (M )n ) = η (g (M n )) ≤ g η (M n )

for every n.

Theorem 3.28. Let A and B be a normed algebras, and let g : A −→ B be an open bounded epimorphism. Then g (Rt (A)) ⊂ Rt (B). ∞

Proof. Let N = {bn }1 be a summable family of B. As g is open, there is a summable family M = {an } in A such that g (M ) = N . It follows from Lemma 3.27 that ρt (g (K)) ≤ ρt (K) for every K ∈ 1 (A). So if a ∈ Rt (A) then, by Corollary 3.25, we obtain that ρt (g (a) N ) ≤ ρt (aM ) = 0 for an arbitrary N ∈ 1 (B). Hence g (a) ∈ Rt (B) by Corollary 3.25. Let A be a normed algebra. Recall that if I is a closed ideal of A, then by a/I (and also by qI (a)) we denote the element a + I of the algebra A/I. By M/I we denote the family {an /I}, for every M = {an } ∈ 1 (A). Theorem 3.29. Let M = {an }∞ 1 be a summable family in a normed algebra A. Then ρt (M ) = ρt (M/I) for each closed tensor quasinilpotent ideal I. In particular, ρt (M ) = ρt (M/Rt (A)). Proof. As clearly ρt (M/Rt (A)) ≤ ρt (M ), it suffices to show the reverse inequality. ∞ Let δ > 0, n ∈ N and M n = {bm }1 . Then for every m there are cm ∈ A and dm ∈ I such that bm = cm + dm and cm ≤ bm /I + 2−m δ.


69

∞ n Let N = {cm }∞ 1 and S = {dm }1 . Since η (N ) ≤ η ( M /I) + δ, then N ∈ 1 (A) . As S = M n − N , we have that S ∈ 1 (I) and that

ρt (N + S) ≤ ρt (N S) by Proposition 3.3. As I ⊂ Rt (A), we have that ρt (N + S) ≤ ρt (N S) = ρt (N ) . by Theorem 3.22. Therefore we obtain that n (3.5)

ρt (M )

= ρt (M n ) = ρt (N + S) ≤ ρt (N ) ≤ η (N ) ≤ η (M n /I) + δ. n

Since δ is arbitrary, then ρt (M ) ≤ η (M n /I) for every n > 0. Taking n-roots and passing to limits, we obtain that ρt (M ) ≤ ρt (M/I). Corollary 3.30. Let A be a normed algebra. Then Rt (A/Rt (A)) = 0. Proof. Let a/Rt (A) ∈ Rt (A/Rt (A)). Then it follows from Theorem 3.29 that ρt ({a} M ) = ρt (M ) for every M ∈ 1 (A). Hence a ∈ Rt (A), and therefore Rt (A/Rt (A)) = 0. Theorem 3.31. Let A be a normed algebra. If I is an ideal of A then Rt (I) = Rt (A) ∩ I. Proof. It is clear that Rt (A) ∩ I ⊂ Rt (I). Let a ∈ Rt (I). For every M ∈ 1 (A), we have that M aM ∈ 1 (I) and then 2

ρt (aM ) = ρt (aM aM ) = 0 by (3.5) and Corollary 3.25. Therefore a ∈ Rt (A) and Rt (I) ⊂ Rt (A) ∩ I.

Note that this result contains Lemma 3.18 and implies that Rt (Rt (A)) = Rt (A) for every normed algebra A. 3.6. Tensor radical algebras and ideals γB A normed algebra A is called tensor radical if the projective tensor product A⊗ is radical for every normed algebra B. It is evident that A is tensor radical if and is tensor radical. If A is tensor radical then its opposite only if its completion A algebra Aop is also tensor radical. An ideal of a normed algebra is called tensor radical if it is a tensor radical algebra. The following result is an easy consequence of associativity and distributivity of tensor product. Proposition 3.32. Let A and B be normed algebras. is tensor radical. (i) If A is tensor radical then A⊗B (ii) If A and B are tensor radical then A ⊕ B is tensor radical.

70

V.S. Shulman and Yu.V. Turovskii The study of deeper properties is based on the following theorem.

Theorem 3.33. For a normed algebra A the following conditions are equivalent. (i) A is tensor radical. (ii) A is tensor quasinilpotent. Proof. (ii) ⇒ (i) follows from Corollary 3.5, taking into account that every element can be represented as M⊗ L for some M ∈ 1 (A) and L ∈ ∞ (B). of A⊗B (i) ⇒ (ii) follows from Theorem 3.1. Indeed, by this theorem, there are a Banach algebra B and L ∈ ∞ (B) such that ρ (M⊗ L) = ρt (M ) for every M ∈ 1 (A). If A is radical, whence ρ (M⊗ L) = 0 for every M ∈ 1 (A). is tensor radical then A⊗B Then ρt (M ) = 0 for every M ∈ 1 (A), i.e., A is tensor quasinilpotent. Corollary 3.34. Every subalgebra of a tensor radical normed algebra is tensor radical. Proof. Follows from Theorem 3.33, since subalgebras of a tensor radical algebra are obviously tensor quasinilpotent. Corollary 3.35. Let A be a normed algebra. If there is a tensor quasinilpotent dense subalgebra B of A then A is tensor quasinilpotent. is also Proof. Indeed, as B is tensor radical by Theorem 3.33, the completion B and A are identified, the algebra A is tensor quasinilpotent tensor radical. As B by Theorem 3.33. Therefore A is tensor quasinilpotent. As a consequence of Corollary 3.23 and Theorem 3.33, for every normed algebra A, Rt (A) is the largest tensor radical ideal of A. Theorem 3.36. Let Abe a normed algebra and a ∈ A. Then a ∈ Rt (A) if and only , for every normed algebra B and b ∈ B. if a ⊗ b ∈ Rad A⊗B B for an arbitrary normed algebra Proof. Let a ∈ Rt (A). Then a ⊗ b ∈ Rt (A) ⊗ is a radical Banach algeB and for every b ∈ B. In the same time, Rt (A) ⊗B (·) B Rt (A) ⊗ bra. Being the image of a bounded homomorphism from Rt (A) ⊗B, consists of quasinilpotent elements of A⊗B. But it is also an ideal of A⊗B. So (·) Rt (A) ⊗ B ⊂ Rad A⊗B and therefore a ⊗ b ∈ Rad A⊗B . Suppose that a ⊗ b ∈ Rad A⊗B for every normed algebra B and b ∈ B. Take B as in Theorem 3.1(ii). Then B has the identity element 1 and there is a family L ∈ ∞ (B) such that ρ (M⊗ L) = ρt (M ) for every M ∈ 1 (A). Since a ⊗ 1 ∈ Rad A⊗B then ρ ((a ⊗ 1) (M⊗ L)) = 0. As (a ⊗ 1) (M⊗ L) = aM⊗ L, we have that ρt (aM ) = 0 for every M ∈ 1 (A). By Theorem 3.21, a ∈ Rt (A). (·)

Proposition 3.37. Let A and B be normed algebras. Then (·) B ⊂ Rt A⊗B ⊂ Rad A⊗B . Rt (A) ⊗


71

Proof. If a ∈ Rt (A) then it follows from Theorem 3.36 that ⊗C a ⊗ b ⊗ c ∈ Rad A⊗B , for every b ∈B and for every normed algebra C and c ∈ C. By the same theorem, is a ⊗ b ∈ Rt A⊗B for every b ∈ B. So the closure of Rt (A) ⊗ B in A⊗B (·) B is generated since Rt A⊗B is closed. But Rt (A) ⊗ contained into Rt A⊗B (·) B ⊂ Rt A⊗B . as a normed algebra by elements of Rt (A) ⊗ B. Hence Rt (A) ⊗ is a Banach algebra, Rt A⊗B ⊂ Rad A⊗B by Corollary 3.20. As A⊗B Proposition 3.38. Let A be a normed algebra and I be an ideal of A. If I and A/I are tensor radical then A is tensor radical. Proof. Let M ∈ 1 (A). As I is tensor radical then, as we mentioned above, the closure I of I in A is also tensor radical. Then ρt (M ) = ρt M/I by Theorem 3.29, but ρt M/I = 0 by Theorem 3.33. So A is tensor radical. Proposition 3.39. Let A be a normed algebra and I be a flexible ideal of A. If A is tensor radical then (I, ·I ) is tensor radical. Proof. Let M = (an )∞ n=1 be a summable family in I. Then M is summable in A and ai1 · · · ain−1 ain η · A (M n ) ≤ η · I (M n ) = I ≤

i1 ,...,in−1

i1 ,...,in

ai1 · · · ain−1 ain I ≤ η · A M n−1 η · I (M ) . A in

Thus ρt|I (M ) = ρt (M ) = 0, where ρt|I is the tensor spectral radius in (I, · I ). Corollary 3.40. Let I and J be flexible ideals of a normed algebra A. If I and J are tensor radical then I ∩ J and I + J are tensor radical with respect to their flexible norms (see Proposition 2.7). Proof. By Corollary 3.25, I and J are contained in Rt (A). Hence the same is true for the ideals I ∩ J and I + J. It follows that they are tensor radical with respect to the norm ·A . By Proposition 3.39, they are tensor radical with respect to their flexible norms. The following result will be often applied in the further sections. 2 , and let Ji ⊂ Ii be ideals Lemma 3.41. Let A1 , A2 be normed algebras, A = A1 ⊗A of Ai , for i = 1, 2. Denote by J the ideal of A generated by J1 ⊗ A2 + A1 ⊗ J2 , and let I be the ideal of A generated by I1 ⊗ A2 + A1 ⊗ I2 . If Ii /Ji are tensor radical then I ⊂ QJ (A).

72


Proof. Let π be a strictly irreducible representation of A such that π (J) = 0. First we show that π(I1 ⊗ A2 ) = 0 and π(A1 ⊗ I2 ) = 0. Assume, to the contrary, that π (I1 ⊗ A2 ) = 0. Hence the restriction τ of π to (·) A2 is strictly irreducible. As τ (J1 ⊗ A2 ) = 0 then τ J1 ⊗ A2 = 0 because I1 ⊗ one may assume that τ is continuous. Moreover, τ induces a strictly irreducible representation of the algebra C := 2 , because the composition of natural maps (I1 /J1 )⊗A 2 −→ I1 ⊗A 2 /J1 ⊗ A2 −→ I1 ⊗ (·) A2 /J1 ⊗ A2 (I1 /J1 )⊗A

is a contractive epimorphism, where J1 ⊗ A2 and J1 ⊗ A2 are the closures of 2 and I1 ⊗ (·) A2 , respectively, and clearly J1 ⊗ A2 ⊂ J1 ⊗ A2 . J1 ⊗ A2 in I1 ⊗A As there exists a non-zero strictly irreducible representation of C then C is not radical, but C is radical in virtue of tensor radicality of I1 /J1 , a contradiction. Hence π (I1 ⊗ A2 ) = 0 and, similarly, π(A1 ⊗ I2 ) = 0. As I lies in the intersection of all primitive ideals of A containing J, so does I. By Proposition 2.1(i), I ⊂ QJ (A). 3.7. Algebras commutative modulo the tensor radical We say that a normed algebra A is commutative modulo the tensor radical if the algebra A/Rt (A) is commutative. An equivalent condition is [a, b] ∈ Rt (A) for all a, b ∈ A. Theorem 3.42. If normed algebras A1 and A2 are commutative modulo the tensor 2. radical then the same is true for A := A1 ⊗A (·)

2 + A1 ⊗ Rt (A2 ) ⊂ Rt (A). Then Proof. By Proposition 3.37, Rt (A1 )⊗A [a1 ⊗b1 , a1 ⊗b2 ] = [a1 , a2 ]⊗b1 b2 + a2 a1 ⊗[b1 , b2 ] ∈ Rt (A) for all a1 , a2 ∈ A1 and b1 , b2 ∈ A2 . Hence [c1 , c2 ] ∈ Rt (A) for all c1 , c2 ∈ A.

Theorem 3.43. Let A1 be a normed algebra, and let A2 be a Banach algebra. If A1 2 is radical. is commutative modulo the tensor radical and A2 is radical then A1 ⊗A 2 and I = Rt (A1 )⊗ (·) A2 . The Banach ideal I of A is radProof. Let A = A1 ⊗A 2 . On the ical, being isometric to the quotient of the radical algebra Rt (A1 )⊗A 2 by other hand, the quotient A/I is topologically isomorphic to (A/Rt (A1 ))⊗A Proposition 2.11. Since A/Rt (A1 ) is commutative and A2 is radical, the algebra 2 is radical by [2, Theorem 4.4.2]. Thus A/I is radical, and A is (A/Rt (A1 ))⊗A radical by Lemma 2.2. 3.8. Relation with joint spectral radius In 1960 Rota and Strang [21] defined a notion of spectral radius for bounded subsets of a Banach algebra. This definition holds for normed algebras (and we already introduced it for countable bounded subsets in Section 3.1). Namely, if K


73

is a bounded subset of a normed algebra A then its joint spectral radius ρ(K) is defined by ρ(K) = inf K n 1/n , n∈N

where the norm of a set is defined as the supremum of the norms of its elements, and the products of sets are defined by KN = {ab : a ∈ K, b ∈ N }. Since K n+k ≤ K n K k for every n, k > 0, one has that ρ(K) = lim K n 1/n . n→∞

(3.11)

Taking n = mk in (3.11) for m = 1, 2, . . ., we observe that ρ(K k ) = ρ(K)k for every bounded K ⊂ A and integer k > 0. It was proved in [25, Theorem 3.5] that the joint spectral radius is a subharmonic function. This means that if λ → K(λ) is an analytic map in a natural sense from a domain D ⊂ C into the set of bounded subsets of A then the function λ → ρ(K(λ)) is subharmonic. Let K be a subset of A, and let F∞ (K) be the set of all bounded families ∞ N = {an }1 with an ∈ K for every n > 0. Clearly F∞ (K) is a metric space with respect to the metric induced by the norm of ∞ (A). In particular, we have that F∞ (A) = ∞ (A). If K is bounded, it follows from [25, Proposition 2.2] that there ∞ is a family L = {bn }1 ∈ F∞ (K) such that ρ(K) = ρ(L). So ρ(K) =

max

N ∈F∞ (K)

ρ(N )

and this allows to obtain some results on joint spectral radius of bounded subsets considering bounded families. The following property is important for our applications: If ρ(K) = 0 then the linear span of K consists of quasinilpotent elements. This result from n[24] can be easily proved by the direct evaluation of the norms of powers for i=1 λi ai , where ai ∈ K. The following result is similar. Lemma ∞ 3.44. Let K be a bounded subset of a normed algebra A. If ρ(K) = 0 then ρ( n=1 λn an ) = 0 for each sequence an ∈ K and each summable sequence λn of complex numbers, where ∞ n=1 λn an are elements of the completion A as usual. ∞ ∞ under the is identified with A Proof. Let M = {λn }1 and L = {an }1 . As C⊗A with λx, we obtain that identification λ⊗x ∞ λn an ≤ ρt (M ) ρ(L) ≤ ρt (M ) ρ(K) = 0 ρ n=1

by Theorem 3.1(i).

We say that a normed algebra A is compactly quasinilpotent [29] if ρ(K) = 0 for each precompact subset K of A. The following statement improves [29, Theorem 4.29] which was proved for Banach algebras.

74


Theorem 3.45. Every compactly quasinilpotent normed algebra is tensor radical. Proof. Let A be a compactly quasinilpotent normed algebra,∞B a Banach algebra, and let x = ∞ n=1 an ⊗bn ∈ A⊗B. One can assume that {an }1 consists of elements of A (see Section 2.4.1), the sequence αn = an is summable, while bn ≤ 1 for every n. It is obvious that there exists a sequence εn → 0 such that λn := αn /εn is summable. Set cn = λ−1 n an for n > 0. Then cn → 0 as n → ∞, so the set N := {cn : n = 1, 2, . . .} is precompact and ρ(N ) = 0 by our assumption. For K = {cn ⊗ bn : n = 1, 2, . . .}, it is easy to check that K k ≤ N k , whence ρ(K) ≤ ρ(N ) = 0. Applying Lemma 3.44, we obtain that ∞ ρ(x) = ρ λn cn ⊗ bn = 0. n=1

consists of quasinilpotent elements. Therefore A⊗B

It was proved in [29] that each normed algebra has the largest compactly quasinilpotent ideal Rc (A). Corollary 3.46. Let A be a normed algebra. Then Rc (A) ⊂ Rt (A) ⊂ A ∩ Rad(A). Proof. The first inclusion follows by Theorem 3.45. of A is tensor By Corollary 3.35, the closure Rt (A) in the completion A As Rt (A) is quasinilpotent. Then Rt (A) consists of quasinilpotent elements of A. an ideal of A consisting of quasinilpotents, Rt (A) ⊂ Rad(A). Therefore we obtain that Rt (A) ⊂ A ∩ Rad(A). 3.9. Compactness conditions It is still an open problem if any radical Banach algebra is tensor radical. We will show here that the answer is positive if A has some compactness properties. In the well-known paper of Vala [32] it was shown that (i) For compact operators a, b on a Banach space X the multiplication operator x −→ axb is compact on B(X). (ii) If the operator x −→ axa is compact then the operator a is compact. This gave a possibility to introduce a notion of a compact element of a normed algebra: an element a of A is compact if Wa := La Ra is a compact operator, where La and Ra are defined by La x = ax and Ra x = xa for every x ∈ A. Similarly, one says that a is a finite rank element of A if Wa has finite rank. Basing on these definitions there were introduced various Banachalgebraic analogues of the class of algebras of compact operators. The most popular one is the class of compact algebras: A is compact if all its elements are compact. Slightly more narrow but much more convenient is the class of bicompact algebras:


75

A is bicompact if La Rb are compact for all a, b ∈ A. Furthermore, A is called an approximable algebra if the set of finite rank elements is dense in A. These classes are closed under passing to ideals and quotients, but in general not stable under extensions. To overcome this obstacle and considerably extend the class of algebras in consideration, let us say that a normed algebra A is hypocompact (respectively, hypofinite) if each non-zero quotient of A has a non-zero compact (respectively, finite rank) element. It is not difficult to check that the class of all hypocompact algebras is closed under extensions, as well as under passing to ideals and quotients. It is not known if it is closed under passing to subalgebras. One can realize a hypocompact algebra as a result of a transfinite sequence of extensions of bicompact algebras, but we will need a close result, see Proposition 3.48 below. Since a quotient of a quotient of A is isomorphic to a quotient of A, the following result is an immediate consequence of the definition of hypocompact algebras. Corollary 3.47. A quotient of a hypocompact normed algebra (by a closed ideal) is hypocompact. A similar result is valid for hypofinite normed algebras. We also need the following result. Proposition 3.48. Let A be a normed algebra. Then A is hypocompact (respectively, hypofinite) if and only if there is an increasing transfinite chain (Jα )α≤β of closed ideals of A such that J0 = 0, Jβ = A, Jα = ∪α m. Proof. Indeed, it follows from Lemma 3.54 that every Riesz operator is quasinilpotent modulo the approximable operators. Our aim now is to show that the class of hypocompact algebras is stable under tensor products. Let ball(A) denote the closed unit ball of A. Recall that Wa = La Ra for every a ∈ A, and if M ⊂ A and N ⊂ B are not subspaces then M ⊗N means only the set {a⊗b : a ∈ M, b ∈ N }, not its linear span. Moreover, if I is a closed ideal of A, then M/I means the set {a/I : a ∈ M } ⊂ A/I. ˆ Lemma 3.56. Let A, B be unital Banach algebras, J a closed ideal in A⊗B, I1 and I2 closed ideals in A and B respectively. Let elements a ∈ A, b ∈ B satisfy the conditions a⊗I2 ⊂ J and I1 ⊗b ⊂ J. If a/I1 and b/I2 are compact elements of A/I1 and B/I2 respectively then (a⊗b)/J is a compact element of (A⊗B)/J. Proof. Note that aAa⊗I2 = (a⊗I2 )(Aa⊗1) ⊂ J and, similarly, I1 ⊗bBb ⊂ J. Choose ε > 0. Let x1 , . . . , xn ∈ ball(A) be such that {Wa xi /I1 : 1 ≤ i ≤ n} is an ε-net in Wa (ball(A/I1 )). This means that for any x ∈ ball(A) there are i ≤ n and x ∈ I1 with Wa x − Wa xi − x < ε. In the same way one finds y1 , . . . , ym ∈ ball(B) such that for each y ∈ ball(B) there are k ≤ m and y ∈ I2 with Wb y − Wb yk − y < ε.

78


Let us check that the set {(Wa xi ⊗Wb yk )/J : i ≤ n, k ≤ m} is a δ-net for the set (Wa (ball(A))⊗Wb (ball(B)))/J, where δ = (a2 + b2 )ε. Indeed, for x ∈ ball(A), y ∈ ball(B), choose i, k as above. Then we obtain that z : = Wa x⊗Wb y − Wa xi ⊗Wb yk = (Wa x − Wa xi )⊗Wb y + Wa xi ⊗(Wb y − Wb yk ) = x ⊗Wb y + Wa xi ⊗y + u⊗Wb y + Wa xi ⊗v where u < ε, v < ε. Since the first two summands belong to J, we conclude that the norm of z/J in A⊗B/J is less than δ. /J. We proved that (Wa (ball(A))⊗Wb (ball(B)))/J is precompact in A⊗B Since ball(A⊗B) is the closed convex hull of the set ball(A)⊗ball(B), then the set (Wa ⊗Wb )(ball(A⊗B))/J is precompact, whence W(a⊗b)/J is compact. Let us say, for brevity, that an element a is compact modulo a closed ideal J if a/J is a compact element of A/J. is hypocomTheorem 3.57. If normed algebras A and B are hypocompact then A⊗B pact. Proof. By Corollary 3.49, one can assume that A and B are complete. Suppose first that A and B are unital. Let J be a proper closed ideal of A⊗B. We have to prove that A⊗B /J has non-zero compact elements. Set I1 = {x ∈ A : x⊗B ⊂ J}. By our assumption, I1 = A (indeed, otherwise J = A⊗B), so there exists an element a ∈ A\I1 which is compact modulo I1 . Set I2 = {y ∈ B : a⊗y ∈ J}. Since a ∈ / I1 then I2 = B. Let b ∈ B\I2 be an element of B compact modulo I2 . By the definition of I2 , a⊗I2 ⊂ J. Furthermore, I1 ⊗b ⊂ J because I1 ⊗B ⊂ J. Hence the assumptions of Lemma 3.56 are satisfied, therefore a⊗b is an element compact modulo J. It is clear that a⊗b ∈ of A⊗B / J by the choice of b. In general it suffices to note that the unitization A1 of a hypocompact algebra 1 , is hypocompact by A is hypocompact and A⊗B, being a closed ideal of A1 ⊗B Corollary 3.60. 3.10. Topological radicals A map P which associates with every normed algebra A a closed ideal P (A) of A is called a topological radical if P (P (A)) = P (A), P (A/P (A)) = 0, P (I) is an ideal of A and P (I) ⊂ P (A) for every ideal I of A, and f (P (A)) ⊂ P (B) for every morphism f : A −→ B. The meaning of the later requirement depends on the specification of morphisms in the different categories whose objects are normed algebras. In the applications below, open bounded epimorphisms are included in the class of morphisms of any such category. The study of topological radicals was initiated by [13]. A topological radical P is called hereditary if P (I) = I ∩ P (A) for any ideal I of each normed algebra A. A normed algebra A is called P -radical if A = P (A) and P -semisimple if P (A) = 0.


79

Note [13] that the Jacobson radical rad : A −→ rad(A) is not a topological radical on the class of all normed algebras, but its restriction Rad to the class of all Banach algebras is a hereditary topological radical. This radical admits different extensions to the class of all normed algebras which are topological radicals (see r for instance [29, Section 2.6]). One of them is the regular extension Rad : A −→ (see [29, Section 2.8]) where A is the completion of A. We already A ∩ Rad A met this radical in Corollary 3.46 and Theorem 3.52. Let Rt denote the map A −→ Rt (A) for every normed algebra A. Theorem 3.58. Rt is a hereditary topological radical in the category of normed algebras morphisms of which are open bounded epimorphisms. Proof. It follows from the results of Sections 3.4 and 3.5.

The same was proved for the map Rc : A −→ Rc (A) (see [29, Theorem 4.25]). Moreover, it was proved in [30, Theorem 3.14] (see also the short communication [26]) that each normed algebra A has a largest hypocompact ideal Rhc (A), and the map Rhc : A −→ Rhc (A) is a hereditary topological radical on the class of normed algebras with open bounded epimorphisms as morphisms. It should be noted that for simplicity the results of [30, Section 3.2] were formulated for Banach algebras, but the proofs did not use the completeness. Theorem 3.59. For every normed algebra A, there exists a largest hypofinite ideal Rhf (A), and the map Rhf : A −→ Rhf (A) is a hereditary topological radical on the class of normed algebras morphisms of which are bounded homomorphisms with dense image. Proof. Similar to the proof in [30, Section 3.2]. One have to replace compact algebras by approximable ones and take into account that a bounded homomorphism with dense image maps finite rank elements into finite rank elements. We note that Rhf -radical algebras are just hypofinite algebras as well as Rhc -radical algebras are hypocompact algebras. Now we note the following useful consequence of the heredity of radicals Rhc and Rhf . Corollary 3.60. Every ideal of a hypocompact (respectively, hypofinite) normed algebra is a hypocompact (respectively, hypofinite) normed algebra.

4. Multiplication operators on Banach bimodules 4.1. Banach bimodules 4.1.1. Elementary operators. Let A, B be Banach algebras and U a bimodule over A, B (shortly (A, B)-bimodule). Then in an obvious way U can be considered as an (A1 , B 1 )-bimodule. We say that U is a normed bimodule if it is a normed space with a norm ·U and aubU ≤ aA uU bB for every a ∈ A1 , b ∈ B 1 and u ∈ U .

80


Let La and Rb be operators on U defined by La x = ax and Rb x = xb for every x ∈ U . By EA,B (U ) we denote the algebra generated by all operators La , Rb . Its elements are called elementary operators on U with coefficients in A, B. If A, B are unital then EA,B (U ) coincides with the algebra EA,B (U ) generated by all La Rb . In the general case EA,B (U ) is an ideal of EA,B (U ) which is an ideal in EA1 ,B 1 (U ), and the latter can be regarded as a unitization of EA,B (U ). Note also that EA,B (U ) = EA1 ,B (U ) + EA,B1 (U ). n be written in the form T = n Clearly operators in EA,B (U ) and EA,B (U ) can i=1 Lai Rbi and, respectively, T = La + Rb + i=1 Lai Rbi , where a, ai ∈ A, b, bi ∈ Bi . One may consider U as a left (B op )-module, where B op is the algebra opposite to B. Then there is a natural homomorphism ψ = ψU from A ⊗γ B op into the algebra B(U ) of all continuous operators on U given by ψ:z=

n i=0

ai ⊗ bi −→

n

Lai Rbi

i=0

for every ai ∈ A and bi ∈ B. Then ψ (z) uU ≤ γ (z) uU

(4.1)

for every u ∈ U , whence ker ψ is closed. Since the image of ψ coincides with EA,B (U ), one may consider EA,B (U ) as a quotient of A ⊗γ B op . This induces the quotient norm ·EA,B , or simply ·E , on EA,B (U ) by ! T E = inf Lai Rbi = T (4.2) ai A bi B : for every T ∈ EA,B (U ). So EA,B (U ) is a normed algebra with respect to ·E . Proposition 4.1. If U is a normed bimodule then ·B(U) ≤ ·E on EA,B (U ), so EA,B (U ) is a normed subalgebra of B (U ) with respect to ·EA,B . Proof. Indeed, if T ∈ EA,B (U ) then T B(U) ≤ T E by (4.1) and (4.2).

In a similar way one can consider EA1 ,B1 (U ) (and therefore EA,B (U )) as a normed subalgebra of B (U ). In the case of unital coefficient algebras A, B we don’t distinguish EA,B (U ) from EA,B (U ). 4.1.2. Multiplication operators. A normed bimodule is called Banach if it is a of a normed bimodule U is a Banach space. It is clear that the completion U . Banach bimodule and that one can identify EA,B (U ) and EA,B U Let U be a Banach (A, B)-bimodule. Let EA,B (U ) be the completion of EA,B (U ) in ·E . It is clear that EA,B (U ) is an algebra of continuous operators on U . The operators in EA,B (U ) are called multiplication operators on U . Again, if U is a Banach bimodule then EA,B (U ) ⊂ B (U ) as usual, and EA,B (U ) is a Banach subalgebra of B (U ).


81

Proposition 4.2. If I and J are flexible ideals of A and B respectively, then EI,J (U ) is a flexible ideal of EA,B (U ), and EI,J (U ) with the norm ·EI,J is a Banach ideal of EA,B (U ). In what follows we often denote the coefficient algebras by A1 , A2 instead of A, B. Theorem 4.3. Let U be a Banach bimodule over normed algebras A1 , A2 . (i) If A1 and A2 are hypocompact then the algebra EA1 ,A2 (U ) is hypocompact. (ii) If at least one of the algebras Ai is tensor radical then EA1 ,A2 (U ) is tensor radical. (iii) If both Ai are commutative modulo the tensor radical then EA1 ,A2 (U ) is commutative modulo the tensor radical. Proof. (i) Indeed, it is easy to see that EA1 ,A2 (U ) is isometric to a quotient of op A1 ⊗A 2 . So it is hypocompact by Theorem 3.57 and Corollary 3.47. op (ii) Since EA1 ,A2 (U ) is isometric to the quotient of A1 ⊗A 2 by the kernel of op 2 into B(U ), the statement follows from the fact that the natural map from A1 ⊗A a quotient of a tensor radical normed algebra is tensor radical (which follows easily from Theorem 3.28). (iii) Arguing as in (ii), we have only to prove that if a normed algebra B is commutative modulo the tensor radical then so is the quotient of B by a closed ideal J. Let qJ : B −→ B/J be the standard epimorphism. Then qJ (Rt (B)) ⊂ Rt (B/J) by Theorem 3.28. Assuming that B/Rt (B) is commutative, we obtain that [a/J, b/J] = qJ ([a, b]) ∈ Rt (B/J)

(a, b ∈ B).

This means that B/J is commutative modulo the tensor radical.

Corollary 4.4. Let U be a Banach bimodule over unital normed algebras A1 , A2 . If A1 and A2 are hypocompact then σB(U) (T ) is (finite or) countable and σB(U ) (T ) = σEA

1 ,A2 (U )

(T )

for every T ∈ EA1 ,A2 (U ). Proof. Indeed, EA1 ,A2 (U ) is a unital Banach subalgebra of B (U ), and σEA ,A (U ) (T ) 1 2 is countable (Theorem 3.50). So σB(U) (T ) = σEA ,A (U ) (T ) by Proposition 2.3. 1

2

Now we consider the sum EA1 ,I2 (U ) + EI1 ,A2 (U ) of Banach subalgebras of B(U ) for closed ideals Ii of Ai . Corollary 4.5. Let Ii be a closed ideal of normed algebra Ai for i = 1, 2. If I1 and I2 are tensor radical then the algebra EA1 ,I2 (U ) + EI1 ,A2 (U ) is tensor radical.

82


Proof. Since EI1 ,A2 (U ) and EA1 ,I2 (U ) are ideals of EA1 ,A2 (U ) and are tensor radical by Theorem 4.3, they are contained in Rt (EA1 ,A2 (U )). Since EA1 ,I2 (U ) + EI1 ,A2 (U ) is a flexible ideal of EA1 ,A2 (U ), it is also tensor radical by Proposition 3.40. Let us consider a more general situation. Theorem 4.6. Let Ji ⊂ Ii be ideals of Ai , i = 1, 2. Suppose that the algebras Ii /Ji are tensor radical. Setting, for brevity, EI = EA1 ,I2 (U ) + EI1 ,A2 (U ) and EJ = EA1 ,J2 (U ) + EJ1 ,A2 (U ) we have that EJ is an ideal of EI and the algebra EI /EJ is tensor radical. Proof. One can clearly assume that the ideals Ii and Ji are closed in Ai for i = 1, 2. (·) (A2 /J2 ) in the Banach alConsider the Banach ideal K1 = (I1 /J1 ) ⊗ γ (A2 /J2 ). As K1 is topologically isomorphic to a quogebra B = (A1 /J1 ) ⊗ tient of (I1 /J1 ) ⊗ (A2 /J2 ) then it is tensor radical. Similarly, the Banach ideal (·) (I2 /J2 ) in B is tensor radical. Then their flexible sum K1 + K2 K2 = (A1 /J1 ) ⊗ in B is tensor radical by Proposition 3.40. (·) A2 +A1 ⊗ (·) I2 )/J1 ⊗ A2 + A1 ⊗ J2 is tensor radical Hence the quotient (I1 ⊗ because it is topologically isomorphic to K1 + K2 by Proposition 2.11. (·) A2 + A1 ⊗ (·) I2 −→ EI /EJ . It Consider now the natural epimorphism ψ : I1 ⊗ is clear that J1 ⊗ A2 + A1 ⊗ J2 ⊂ ker ψ. So there is a bounded homomorphism from (·) A2 + A1 ⊗ (·) I2 )/J1 ⊗ A2 + A1 ⊗ J2 onto EI /EJ . This epimorphism is open (I1 ⊗ by the Banach Theorem. Therefore EI /EJ is tensor radical by Theorem 3.28. The proved result is important for applications in Section 6. Note that Corollary 4.5 can be obtained from Theorem 4.6 if one takes J1 = J2 = 0. In the following result we preserve notation of Theorem 4.6. Corollary 4.7. Let I and J be as in Theorem 4.6. Then EI ⊂ QEJ (EA1 ,A2 (U )). Thus if T ∈ EI then, for every ε > 0, there are m ∈ N and elementary operators Sn ∈ EJ on U such that T n − Sn E < εn for every n > m. Proof. Follows from Proposition 2.1.

4.2. Operator bimodules Let us consider the case that A1 , A2 are the algebras B(Y ), B(X) of all bounded operators on Banach spaces X, Y . Let U be a normed subspace of B(X, Y ) of all bounded operators from X to Y with the natural bounded (B(Y ), B(X))-bimodule structure; we refer to it as a normed subbimodule of B(X, Y ) or, simply, a normed operator bimodule. The latter means that U is supplied with its own norm ·U ≥ ·B = · and axbU ≤ a xU b


83

for all a ∈ B(Y ), b ∈ B(X), x ∈ U . It is easy to see that if U is non-zero then U contains all finite rank operators. We also may assume that xU = x for every rank one operator x ∈ U . When U is complete in ·U , one says that U is a Banach operator bimodule. In this case, for brevity, we also denote EB(Y ),B(X) (U ) by B∗ (U ) and call its elements (B)-multiplication operators on U . It is clear that B∗ (U ) is a Banach subalgebra of B (U ) with respect to the norm ·B∗ = ·EB(Y ),B(X) . Operator bimodules are closely related to operator ideals. If U is a Banach operator ideal in the sense of [20], e. g. the ideal K of compact operators or the ideal N of nuclear operators then each component U = U(X, Y ) of U is a Banach operator bimodule. It can be proved that all Banach operator bimodules can be obtained in this way. 4.2.1. Semicompact multiplication and (K)-multiplication operators. The algebras Ai are semisimple so they have no radical ideals. But they can have pairs of ideals Ji ⊂ Ii with radical quotients Ii /Ji . Indeed, Lemma 3.54 shows that this is the case if we take the ideals K(X), K(Y ) for Ii , and the ideals F (X), F (Y ) for Ji . The possibility to use Theorem 4.6 is important for further applications. Before formulate the corresponding corollaries it will be convenient to introduce special terminology. Namely we set 1 (U ) = EB(Y ),K(X) (U ) + EK(Y ),B(X) (U ), K 2

F 21 (U ) = EB(Y ),F (X) (U ) + EF (Y ),B(X) (U ). 1 (U ) is taken here as the sum of Banach ideals EB(Y ),K(X) (U ) and Note that K 2 EK(Y ),B(X) (U ) in B∗ (U ) with respective flexible norm (see Proposition 2.7). Thus 1 (U ) consists of multiplication operators T on U of the form ∞ Lai Rbi such K i=1 2 that i ai bi < ∞ and at least one of the operators ai or bi is compact for every i. The norm T K 1 is equal to inf i ai bi for all such representations 2

1 (U ) semicompact multiplication operators. of T . We call operators in K 2 Similarly, operators in F 12 (U ) are called semifinite elementary operators. n They are just the elementary operators i=1 Lai Rbi where ai or bi is a finite rank operator for each i. As a concrete application of Theorem 4.6, we obtain the following Corollary 4.8. Let U ⊂ B (X, Y ) be a Banach operator bimodule. Then the algebra 1 (U ) /F 1 (U ) is tensor radical. K 2 2 As a consequence we have the following Corollary 4.9. Let U ⊂ B (X, Y ) be a Banach operator bimodule. Then 1 (U ) ⊂ QF (U ) (B∗ (U )). K 2

1 2

84


Since the norm in B∗ (U ) majorizes the operator norm in B(U ), we also obtain the following result. Corollary 4.10. Let T be a semicompact multiplication operator on a Banach operator bimodule U . Then for each ε > 0 there are m ∈ N and semifinite elementary operators Sn on U such that T n − Sn B(U) < εn for every n > m. ∗ (U ) of (K)-multiplication The other useful ideal in B∗ (U ), namely the ideal K operators, is defined by ∗ (U ) = EK(Y ),K(X) (U ). K In many cases, for instance when the norm of U coincides with the operator one, ∗ (U ) consists of compact operators on U . It is clear that K ∗ (U ) is a Banach K 1 (U ) and B∗ (U ) with respect to the norm · = · ideal of K K∗ EK(Y ),K(X) . 2 ∗ (U ) is a bicompact Banach algebra for every Banach operator Proposition 4.11. K bimodule U . ∗ (U ) is topologically isomorphic to a quotient of the projective Proof. Indeed, K op tensor product of bicompact algebras K (Y ) and K (X) which is bicompact itself by Lemma 3.56. Corollary 4.12. Let U be a Banach operator bimodule. Then σB(U ) (T ) is at most countable and σB(U ) (T ) ∪ {0} = σK ∗ (U ) (T ) ∪ {0} = σB∗ (U ) (T ) ∪ {0} for every ∗ (U ). T ∈K ∗ (U ) is a Banach subalgebra of B (U ) and is a Banach ideal of Proof. Indeed, K B∗ (U ). Since σK ∗ (U) (T ) is countable by [1, Theorem 4.4], then it is easy to see that σB(U ) (T ) ∪ {0} = σK ∗ (U ) (T ) ∪ {0} by Proposition 2.3. Further, we have that σK ∗ (U ) (T ) ∪ {0} = σB∗ (U ) (T ) ∪ {0} by Remark 2.4. 4.2.2. (N )-multiplication operators and trace. The norms and spectra of elementary operators were studied in many works. Here we would like to mention a simple formula for their traces. Let N be the operator ideal of nuclear operators. Recall that every operator∗ a ∈ N (X, Y ) has a representation fi ⊗ xi with fi xi < ∞ for fi ∈ X and xi ∈ Y . The nuclear norm ·N is given by ! fi ⊗ xi = a . fi xi : aN = inf γ Y is identified with So, as is well known, the projective tensor product X ∗ ⊗ N (X, Y ). If X = Y , then the trace of a is defined by trace (a) = fi (xi ) .


85

Let U ⊂ B(X, Y ) be a Banach operator bimodule. One can define the ideal N∗ (U ) of (N )-multiplication operators by ∗ (U ) = EN (Y ),N (X) (U ) N with respect to the norm ·N ∗ = ·EN (Y ),N (X) . Proposition 4.13. Every (N )-multiplication operator T on U is nuclear. If T = Lai Rbi with ai N bi N < ∞ for ai , bi ∈ N , then trace (ai ) trace (bi ) . trace (T ) = i

Proof. Assume first that a = f ⊗ x and b = g ⊗ y are rank one operators. For every u ∈ U , we obtain that (f ⊗ x) u (g ⊗ y) = f (uy) g ⊗ x. It is clear that the map u −→ f (uy) is a bounded linear functional on U . Indeed, |f (uy)| ≤ f u y ≤ (f y) uU . In the same time, g ⊗ x ∈ U for every g ∈ X ∗ and x ∈ Y . So La Rb is a rank one operator. This also shows that if ai and bi are nuclear then Lai Rbi is nuclear, because an absolutely convergent series of nuclear operators is nuclear. Therefore T is nuclear by the same reason. Clearly trace (Lf ⊗x Rg⊗y ) = f ((g ⊗ x) y) = f (x) g (y) = trace (f ⊗ x) trace (g ⊗ y) . Therefore, for nuclear ai = fj ⊗ xj and bi = gk ⊗ yk , we obtain that trace(Lfj ⊗xj Rgk ⊗yk ) = trace(fj ⊗ xj )trace(gk ⊗ yk ) trace(Lai Rbi ) = j,k

j,k

= trace(ai )trace(bi ), whence trace (T ) =

trace (Lai Rbi ) =

trace (ai ) trace (bi ) .

4.3. Some constructions related to multiplication operators 4.3.1. Integral operators. Let U ⊂ B(X, Y ) be a Banach operator bimodule. Assume first that U is reflexive as a Banach space. Let (Ω, μ) be a measure space, and B(Y ) let L2 (Ω, μ) be the space of all measurable B(Y )-valued functions a : ω −→ a(ω) with the norm 2 aL2 = a(ω) dμ < ∞. Ω

B(Y )

B(X)

For any two functions ω −→ a(ω) ∈ L2 (Ω, μ) and ω −→ b(ω) ∈ L2 (Ω, μ), one can define an operator Ta,b on U by means of the Bochner integral [15, Section

86


3.3.7]

Ta,b(x) =

a(ω)xb(ω)dμ. Ω

K(Y )

(Ω, μ) if replace B (Y ) by K (Y ), and consider Note that one may define L2 K(Y ) B(Y ) L2 (Ω, μ) as a subspace of L2 (Ω, μ). Proposition 4.14. Let U ⊂ B(X, Y ) be a Banach operator bimodule, and let U be reflexive as a Banach space. Then Ta,b ∈ EB(Y ),B(X) and Ta,b EB(Y ),B(X) ≤ K(Y )

aL2 bL2 . If in particular ω −→ a(ω) ∈ L2 (Ω, μ) (respectively, ω −→ b(ω) ∈ K(X) (Ω, μ)) then Ta,b ∈ EK(Y ),B(X) and Ta,bEK(Y ),B(X) ≤ aL2 bL2 (respectL2 ively, Ta,b ∈ EB(Y ),K(X) and Ta,b ≤ a b ). EB(Y ),K(X)

L2

L2

B(Y )

Proof. Let an (ω) ∈ L2 (Ω, μ) be a sequence of simple functions that tend to B(Y ) a(ω) in norm of L2 (Ω, μ). Clearly Ta,b is the limit of operators Tan ,b in the norm topology of B(U ). Each an is a finite sum of functions ki ϕi (t), where ki ∈ B(Y ) and ϕi is the characteristic function of a measurable set Λi ⊂ Ω. Hence Lki Rti Tan ,b = where ti =

i

" Λi

b(t)dμ ∈ B (X). Therefore Tan ,b ∈ EB(Y ),B(X) and Tan ,b EB(Y ),B(X) ≤ an L2 bL2 .

It follows from this that the sequence of operators Tan ,b is fundamental in EB(Y ),B(X) , so it tends to some element T ∈ EB(Y ),B(X) , and T EB(Y ),B(X) ≤ aL2 bL2 . By the above, T = Ta,b and we are done. The other statements are proved similarly. K(Y )

K(X)

B(Y )

B(X)

Let a ∈ L2 (Ω, μ), b ∈ L2 (Ω, μ), s ∈ L2 (Ω, μ), t ∈ L2 (Ω, μ). Then the operator Ta,b,s,t defined on a Banach operator bimodule U ⊂ B(X, Y ) by the formula Ta,b,s,t (x) = a(ω)xt(ω)dμ + s(ω)xb(ω)dμ, I

I

is called an integral semicompact operator. Indeed, it follows from Proposition 4.14 that this operator is semicompact multiplication operator. If we wish to remove the restriction of reflexivity of U and still have that 1 (U ), we should impose continuity conditions which allow Ta,b,s,t belongs to K 2 us to deal with Riemann integral sums (see for instance [15, Section 3.3.7]). For brevity we will formulate the corresponding result in a form which is far from the most general.


87

Theorem 4.15. Let U ⊂ B(X, Y ) be a Banach operator bimodule, and let (Ω, μ) = (I, μ), where μ is a regular measure on an interval I ⊂ R. Then every integral 1 (U ) and semicompact operator Ta,b,s,t belongs to K 2 Ta,b,s,t K 1 (U ) ≤ aL2 tL2 + bL2 sL2 . 2

4.3.2. Matrix multiplication operators. Let (Tji )nj,i=1 be a matrix of multiplication operators on a Banach operator bimodule U ⊂ B(X, Y ). It defines an operator T = [Tij ] on U (n) by the formula Tij xj . T (x1 , . . . , xn ) = (y1 , . . . , yn ) where yi = j

Let us denote the algebra of all such operators by Mn (EB(X),B(Y ) (U )). Also, by 1 (U )) we denote the ideal of Mn (EB(X),B(Y ) (U )) which consists of all operMn (K 2 1 (U ) for 1 ≤ i, j ≤ n. In a similar way we define the ators T = [Tij ] with Tij ∈ K 2

1 (U )). The closure Mn (F 1 (U )) of Mn (F 1 (U )) in subspace Mn (F 12 (U )) of Mn (K 2 2 2 1 (U )) is an ideal of Mn (K 1 (U )). Mn (K 2 2 Theorem 4.16. Let U ⊂ B(X, Y ) be a Banach operator bimodule. Then the algebra 1 (U ))/Mn (F 1 (U )) is tensor radical. Mn (K 2 2 1 (U )) is topologically isomorphic to Mn ⊗γ (K 1 (U )), Proof. The algebra Mn (K 2 2

where Mn is the algebra of n × n matrices. Furthermore, the algebra Mn (F 12 (U ))

is topologically isomorphic to Mn ⊗γ F 12 (U ). Hence, for every Banach algebra A, we have that 1 (U ) /Mn F 1 (U ) 1 (U ) / Mn ⊗γ F 1 (U ) ⊗A ∼ Mn K ⊗ ⊗A M K = n γ 2 2 2 2 ∼ 1 (U )/F 1 (U ) ⊗A = Mn ⊗γ K 2 2 ∼ 1 (U )/F 1 (U ) ⊗ (Mn ⊗γ A) = K 2 2 1 (U )/F 1 (U ) is tensor radical by CorolThe latter algebra is radical because K 2 2 lary 4.8.

5. Multiplication operators on algebras satisfying compactness conditions In this section we consider elementary and multiplication operators in the most popular meaning: as elementary and multiplication operators with coefficients in a Banach algebra acting on the algebra itself. In terms of the previous section, we consider the case A1 = A2 = U = A, that is we regard A as an A-bimodule. For brevity we remove the indication of a bimodule in our standard notation for the

88


multiplication algebra: we write EA,A instead of EA,A (A) (taking the occasion to use EI,J for ideals I, J ⊂ A). Furthermore, we denote the algebra of all elementary operators on A by E(A) instead of EA,A (A). To make our assumptions more concrete we impose various compactness conditions on A. As we know, even the weakest of them, the hypocompactness of A, implies that Rad(A) coincides with Rt (A). 5.1. Multiplication operators on algebras commutative modulo the radical Since radical hypocompact Banach algebras are tensor radical, we can apply results of Section 4. Corollary 5.1. If A is a hypocompact Banach algebra then the algebra ERad(A),A + EA,Rad(A) is tensor radical and hypocompact. Proof. The first statement follows from Corollary 4.5. Arguing as in the proof of Theorem 4.3 with using Theorem 3.57 and Corollary 3.47, we prove the second statement. n In particular all elementary operators La + Rb + i=1 Lai Rbi with ai or bi in Rad(A) for each i are quasinilpotent elements of EA,A . Since the norm in EA,A majorizes the operator norm, they are quasinilpotent operators on A. Below by spectra of elementary operators we mean their spectra in the algebra B(A) of all bounded operators E(A)1 of E(A) consists n on A. Clearly the unitization 1 of elements of the form i=1 Lai Rbi where ai , bi ∈ A . n Theorem 5.2. Let A be a hypocompact Banach algebra. If u = i=1 Lai Rbi ∈ m E(A)1 , v = j=1 Lcj Rbj ∈ E(A)1 and all commutators [ai , cj ] and [bi , dj ] belong to Rad(A) then σ(u + v) ⊂ σ(u) + σ(v) (5.1) and σ(uv) ⊂ σ(u)σ(v). (5.2) op and Proof. Let us denote A1 by B and Rad(A) by J for brevity. Let C = B ⊗B op op E = J⊗B +B⊗J . As J = Rt (A) E ⊂ Rad(C). by Theorem 3.52, we have that Setting u = a ⊗b and v = c ⊗b , we have that [u , v ] ∈ E (because i j i i j j [a⊗b, c⊗d] = [a, c]⊗bd + ca⊗[b, d]). So these elements commute modulo the radical of C. Let φ : C −→ B(A) be the homomorphism sending a ⊗ b to La Rb for every a, b ∈ B. Then the algebra D = φ(C) supplied with the norm of the quotient C/ ker φ is a Banach subalgebra of B(A). The elements u = φ(u ) and v = φ(v ) commute modulo φ(Rad(C)) ⊂ Rad(D). Hence σD (u + v) ⊂ σD (u) + σD (v)

(5.3)

σD (uv) ⊂ σD (u)σD (v),

(5.4)

and where σD (x) denotes the spectrum of x ∈ D with respect to D.


89

As C is hypocompact by Theorem 3.57, and D is isomorphic to a quotient of C, then D is hypocompact by Corollary 3.47. By Theorem 3.50, σD (u) and σD (v) are finite or countable. Using Proposition 2.3, we get that σD (u) = σ(u) and σD (v) = σ(v). So the inclusions (5.3) and (5.4) imply (5.1) and (5.2). Let A be a Banach algebra. Recall that the center modulo the radical or “Rad-center” ZRad (A) is the set {a ∈ A : [a, x] ∈ Rad(A) for all x ∈ A}. Corollary 5.3. If A is a hypocompact Banach algebra and u ∈ LZRad (A) RA + LA RZRad (A) then inclusions (5.1) and (5.2) hold for all v ∈ E(A). Let us call a subalgebra B of a Banach algebra spectrally computable if inclusions (5.1) and (5.2) hold for all elements u, v ∈ B. Corollary 5.4. If A is a hypocompact Banach algebra commutative modulo the radical then E(A) is a spectrally computable subalgebra of B (A). Remark 5.5. In virtue of Corollary 4.4 and by continuity of the spectrum on operators with countable spectra, the previous results as well as the results of Section 5.2 can be extended to multiplication operators. But we prefer to present them in less general and more traditional setting of elementary operators. 5.2. Engel algebras A Banach Lie algebra L is called Engel if all operators adL (a) : x → [a, x] on L are quasinilpotent. This is a natural functional-analytic extension of the class of nilpotent Lie algebras because the latter can be defined as Lie algebras for which all operators adL (a) are nilpotent of some restricted order [38]. A Banach algebra A is said to be Engel if it is Engel as a Banach Lie algebra that is if all operators La − Ra are quasinilpotent. It is proved in [28, Proposition 5.21] (and can be easily deduced from a more general result of Aupetit and Mathieu [3]) that all Engel Banach algebras are commutative modulo the radical. We call A strongly Engel if n n σ Lai Rbi ⊂ σ ai b i i=1

i=1

for all ai , bi ∈ A1 and n ∈ N. It will be shown below that for hypocompact Banach algebras these notions coincide. Theorem 5.6. Let A be a hypocompact Banach algebra commutative modulo the radical. Suppose that A is generated (as a Banach algebra) by a subset M such that the operators La − Ra are quasinilpotent for all a ∈ M . Then A is strongly Engel. Proof. By Corollary 5.4, the algebra E(A) is spectrally computable. Using this fact, it can be easily shown that the set E of all a ∈ A, for which σ(La − Ra ) = {0} is a subalgebra of A. Indeed, if a, b ∈ E then σ(La+b − Ra+b ) = σ(La − Ra + Lb − Rb ) ⊂ σ(La − Ra ) + σ(Lb − Rb ) = {0},

90


so a + b ∈ E. Furthermore, σ(Lab − Rab ) = σ(La (Lb − Rb ) + (La − Ra )Rb + Rab−ba ) ⊂ σ(La )σ(Lb − Rb )) + σ(La − Ra )σ(Rb ) + σ(Rab−ba ) = {0} whence ab ∈ E. Since A is generated by M and M ⊂ E, the subalgebra E is dense in A. Since A is hypocompact, its elements have countable spectra by Theorem 3.50 and therefore the spectra of all operators La − Ra are countable. Hence they are the points of continuity of the spectral radius. It follows that E is closed whence E = A. We proved that A is an Engel algebra. n To seethat A is strongly Engel, note that operator i Lai Rbi can be an n n written as i Lai (Rbi − Lbi ) + Lc , where c = i ai bi . Since E(A) is spectrally computable and σ(Rbi − Lbi ) = {0}, we obtain that n σ Lai Rbi ⊂ σ(Lc ) ⊂ σ(c). i

Corollary 5.7. Let A be a hypocompact Banach algebra generated by an Engel closed Lie subalgebra L. Then A is a strongly Engel Banach algebra commutative modulo the radical. Proof. Without loss of generality, we consider the case when A is unital and generated as a Banach algebra by L and the identity element 1. Let us show first that all operators La − Ra with a ∈ L are quasinilpotent on A (by our assumptions, they are quasinilpotent on L). Indeed, they are bounded derivations of A and their spectra are countable (because spectra of elements of A are countable by Theorem 3.50). By [28, Corollary 3.7], they are quasinilpotent on the closed subalgebra of A generated by L, that is on A. Taking into account Theorem 5.6, we have only to prove that A is commutative modulo the radical. Let π be a strictly irreducible representation of A on X. We will obtain a contradiction assuming that dim(X) > 1. Changing A by A/ ker π, one may suppose that π is faithful. We already know that spectra of elements of A are countable. Now we claim that the spectra of elements of L are one-point. Indeed, if σ (a) is not a singleton for some element a ∈ L then it is not connected and, by [28, Proposition 3.16], a has a non-trivial Riesz projection p commuting with L. Hence p is in the center of A, which is impossible, because A is a primitive Banach algebra. Define the function h : L −→ C by h (a) = λ if σ (a) = {λ}, for every a ∈ L. We claim that h is a character of L, i.e., h is a bounded linear functional on L that vanishes on [L, L]. Indeed, by using Proposition 3.48, one can find a proper closed ideal J of A such that A/J is a compact Banach algebra. As 1/J is a compact element, then clearly it is a finite rank element. Hence A/J is finite dimensional. As σ (a/J) ⊂ σ (a) for every a ∈ L, then one can define the function g : L/J −→ C by g (a/J) = λ if σ (a/J) = {λ} for every a ∈ L. It is clear that g (a/J) = h (a) for every a ∈ L and L/J is a nilpotent Lie subalgebra of A/J, whence A/J is


91

commutative modulo the radical by the Engel theorem and g is a character of L/J. So h is a character of L. Now, replacing every element a of L with σ (a) = {λ} by a − λ, one may assume that L consists of quasinilpotent elements. Then G = exp(L) = {exp(a) : a ∈ L} is a group by [37], and σ(b) = {1} for each b ∈ G. It follows that r(M ) = 1 for every precompact subset M of G. By Corollary 3.51, ρ(M ) = 1. In particular, ρ(M ∪ {1, exp (λa)}) = 1

(5.5)

for every a ∈ L and λ ∈ C. Choose an arbitrary element a ∈ L and define the function f on C by f (λ) = ρ(M (exp(λa) − 1)/λ) for a fixed precompact subset M of G. This function is subharmonic by [25, Theorem 3.5] and tends to zero when λ → ∞, because ρ (M (exp(λa) − 1)) ≤ ρ((M ∪ {1}) exp(λa) − M ∪ {1}) ≤ ρ((M ∪ {1, exp(λa)})2 − (M ∪ {1, exp(λa)})2 ) 2 ≤ ρ 2 abs (M ∪ {1, exp(λa)}) 2 = 2ρ (M ∪ {1, exp(λa)}) 2

= 2ρ (M ∪ {1, exp(λa)}) = 2, by (5.5), where abs (S) denotes the absolutely convex hull of a bounded set S ⊂ A and the equality ρ (abs (S)) = ρ (S) [25, Proposition 2.6] easily follows from the characterization of ρ (S) as infimum of S when · runs over all algebra norms equivalent to given one. Hence f is constant and, moreover, f (λ) = 0 for all λ. In particular, f (0) = 0, whence it is easy to see that ρ(aM ) = 0. Now if an element x belongs to the linear span of M then ρ(ax) = 0 by Lemma 3.44. Since linear span lin(G) of G is a subalgebra of A and the closure of lin(G) contains L, we conclude that lin(G) is dense in A. The previous argument shows that ρ(ax) = 0 for all x ∈ lin(G). Since spectra of elements of A are countable, the spectral radius is continuous on A, whence ρ(ax) = 0 for all x ∈ A. This means that a ∈ Rad(A). So L ⊂ Rad(A) and the closed algebra generated by L is radical. Then dim(X) = 1, a contradiction.

92


5.3. Generalized multiplication operators It follows from the above that if A is a radical hypocompact algebra then all operators in E (A) and, more generally, in EA,A are quasinilpotent. Here we discuss the possibility to extend this result to operators in the norm-closure Mul(A) of E (A) in B(A). Note first of all that it is an open problem if Mul(A) is radical even for bicompact A (and even for the case that A is a radical closed algebra of compact operators). We call elements of Mul(A) generalized multiplication operators. Theorem 5.8. If A is a compact algebra and J = Rad(A) then the closed ideal I generated by LJ RA ∪ LA RJ is contained in Rad(Mul(A)). As a consequence, LJ + RJ + I consists of quasinilpotents. Proof. Since A is compact, La2 Rb is compact for any a, b ∈ A. Indeed, since La Rb + Lb Ra = 1/2(La+b Ra+b − La−b Ra−b ) is a compact operator, the same is true for La2 Rb = La(La Rb + Lb Ra ) − (La Ra )Lb . Now it follows from Corollary 5.1 that if b ∈ J then La2 Rb T is a compact quasinilpotent operator for every elementary operator T on A. By continuity of the spectral radius, the same is true for all T ∈ Mul(A). Hence La2 Rb ∈ Rad(Mul(A)). By the Nagata–Higman theorem (for n = 2), every product a1 a2 a3 of elements of A can be represented as a finite combination of elements of form x2 y and uv 2 . Since clearly Lx2 y Rb , Luv2 Rb ∈ Rad(Mul(A)), then LA3 RJ ⊂ Rad(Mul(A)). This implies that (LA RJ )3 ⊂ Rad(Mul(A)). As the Jacobson radical of a Banach algebra is closed, we obtain that LA RJ consists of quasinilpotent operators. Since LA RJ is an ideal of Mul(A), then LA RJ ⊂ Rad(Mul(A)). We proved that LA RJ ⊂ Rad(Mul(A)). Similarly, we have that LJ RA ⊂ Rad(Mul(A)). Hence I ⊂ Rad(Mul(A)). Then LJ + RJ + I consists of quasinilpotents if and only if LJ /I + RJ /I consists of quasinilpotents in Mul(A)/I. The last is obvious because La /I and Rb /I commute for every a, b ∈ A and are quasinilpotents for every a, b ∈ J. Let us denote by Mul2 (A) the closed subalgebra of Mul(A) generated by all operators La Rb , where a, b ∈ A. Corollary 5.9. If a radical Banach algebra A is compact then Mul2 (A) ⊂ Rad(Mul(A)). As a consequence, the algebra E(A)+Mul2 (A) consists of quasinilpotent operators.


93

5.4. Permanently radical algebras Let us call a class P of Banach algebras permanent if for each A ∈ P and each bounded homomorphism f : A −→ B with dense image, the algebra B is also in P. Examples of permanent classes are commutative algebras, separable algebras, finite-dimensional algebras, amenable algebras, algebras with bounded approximate identities. An example of Dixon [13, Example 9.3] shows that the class of all radical Banach algebras is not permanent. We say that a Banach algebra A is permanently radical if for every bounded homomorphism f : A −→ B the closure of the image f (A) in B is a radical Banach algebra. It follows from (i) of the following theorem that the class of all permanently radical Banach algebras is permanent. We also show that it is extension stable. Theorem 5.10. Let A be a Banach algebra. (i) If A is permanently radical then so is g (A) for every bounded homomorphism g : A −→ B of Banach algebras. (ii) If a closed ideal J and the quotient A/J of A are permanently radical then A is permanently radical. (iii) If {Iα }α∈Λ is an increasing net of permanently radical closed ideals in A and ∪α∈Λ Iα is dense in A then A is permanently radical. Proof. (i) Let C = g (A) and f : C −→ D be a bounded homomorphism of Banach algebras with f (C) = D. Then f ◦g is a bounded homomorphism A −→ D with dense image. If A is permanently radical then D is radical. (ii) Let f : A −→ B be a bounded homomorphism with f (A) = B. Then I := f (J) is a radical ideal of B. Hence I ⊂ Rad(B), whence g = qRad(B) ◦f is a bounded homomorphism of A into C = B/Rad(B) and I ⊂ ker g. Thus there is a bounded homomorphism h : A/J → C such that g = h◦qJ . As C = h(A/J), C is radical. Then C = 0, whence B is radical. (iii) If f : A → B is a bounded homomorphism with dense image then all f (Iα ) are radical ideals of B. Hence f (Iα ) ⊂ Rad(B), for each α, and f (A) ⊂ Rad(B) by density. Thus B = Rad(B). Remark 5.11. It is not clear if (ii) may be reversed. It follows from (i) that a quotient of a permanently radical Banach algebra is permanently radical, but what one can say about ideals? Clearly the class of all permanently radical Banach algebras contains all radical commutative Banach algebras and all finite-dimensional radical algebras. Theorem 5.12. Every radical hypofinite Banach algebra A is permanently radical. Proof. Let us show first that each topologically irreducible representation π of A on a Banach space X is zero. Indeed, assume that π = 0 and let J = ker π, then A/J contains a non-zero finite rank element a/J. Since π(a) = 0 there is 0 = x ∈ X with π(a)x = 0, whence π(A)π(a)x is a dense subspace of X. Since π(a)π(A)π(a)x =

94


π(aAa)x is a finite-dimensional subspace, we conclude that dim(π(a)X) < ∞. Let I = π(A) ∩ F(X), this is a non-zero ideal of π(A). Hence I has no closed invariant subspaces. On the other hand, I consists of nilpotent operators (indeed, if a finite rank operator is the image of a quasinilpotent element under a representation then it is nilpotent). By the Lomonosov Theorem [18] (or by an earlier result of Barnes [5] ), I has an invariant subspace. This contradiction shows that π = 0. Let now f : A −→ B be a continuous homomorphism with dense image. If π is a strictly irreducible representation of B then π ◦ f is a topologically irreducible representation of A whence π ◦ f = 0 and π = 0. This shows that B is radical. Corollary 5.13. If A is a hypofinite Banach algebra then Rad(A) is permanently radical. Proof. Rad(A) is a hypofinite Banach algebra, because it is an ideal of A (see Corollary 3.60). So apply Corollary 5.12. It would be convenient to formulate a result established in the proof of Theorem 5.12 as follows. Proposition 5.14. Each topologically irreducible representation of a radical hypofinite Banach algebra is trivial. Is any radical bicompact Banach algebra permanently radical? Note that the positive answer would imply that all hypocompact radical Banach algebras are permanently radical. But even if the answer is affirmative it needs another approach because the following result shows that Proposition 5.14 doesn’t extend to radical hypocompact algebras. Theorem 5.15. There is a radical bicompact, singly generated Banach algebra A with a non-trivial topologically irreducible contractive representation by bounded operators. Proof. Let T be a quasinilpotent operator on a Banach space X without non-trivial closed invariant subspaces (the existence of such operators is a famous example by Read [19]). Let B be the subalgebra of B(X) generated by T . It follows from Bonsall’s theorem [8, Theorem 3] that there is an algebra norm · on B such that 1) a ≤ a for each a ∈ B, 2) the completion A of B in · is a Banach subalgebra of B (X), 3) the element b of A corresponding to T is compact. Since A is generated by b, it is a bicompact, singly generated Banach algebra. As every compact element of a Banach algebra has countable spectrum by [1, Theorem 4.4], σA (b) = σ (T ) by Proposition 2.3(ii). Hence b is a quasinilpotent element of A, and A is radical. As A is embedded into B (X), let π be the natural representation of A by bounded operators on X. Then π(b) = T , and π is topologically irreducible and contractive.


95

Theorem 5.16. If A is a compact Banach algebra and Rad(A) is permanently radical, then LRad(A) ∪ RRad(A) ⊂ Rad(Mul(A)). Proof. Let I = Rad(Mul(A)), C = Mul(A)/I and q = qI . Define φ : Rad(A) → C by φ(a) = q(La ) for any a. Then the algebra φ(Rad(A)) is radical. For any a ∈ Rad(A) and T ∈ E(A), we have that La T ∈ LRad(A) + LRad(A) RA ⊂ LRad(A) + Rad(Mul(A)) by Theorem 5.8. It follows that q(La T ) ∈ φ(Rad(A)). By continuity, the same is true for all T ∈ Mul(A). Thus all q(La T ) are quasinilpotent. This shows that LRad(A) Mul(A) consists of quasinilpotents, whence LRad(A) ⊂ Rad(Mul(A)). Similarly, we have that RRad(A) ⊂ Rad(Mul(A)). Corollary 5.17. If A is an approximable Banach algebra then LRad(A) ∪ RRad(A) ⊂ Rad(Mul(A)). Proof. Clearly A is compact. Furthermore, Rad(A) is permanently radical by Corollary 5.13. Corollary 5.18. If A is an approximable Banach algebra and A is commutative modulo Rad(A), then Mul(A) is commutative modulo Rad(Mul(A)). Proof. For all a, b ∈ A, [La , Lb ] = L[a,b] ∈ LRad(A) ⊂ Rad(Mul(A)) and, similarly, [Ra , Rb ] ∈ Rad(Mul(A)). Since also [La , Rb ] = 0 ∈ Rad(Mul(A)), we get that Mul(A)/Rad(Mul(A)) is commutative. 5.5. Chains of closed ideals Now we consider invariant subspaces of the algebras of elementary operators. It was proved by Wojty´ nski [36] that the well-known problem of the existence of a non-trivial closed ideal in a radical Banach algebra has the positive answer if the algebra has a non-zero compact element. The proof of this fact, based on the invariant subspace theorem for Volterra semigroups is given in [31]. The following theorem presents another proof and a slightly more general formulation of this result. Recall that a central multiplier on a Banach algebra A is a bounded linear operator on A commuting with left and right multiplications. Theorem 5.19. If a radical Banach algebra A has a non-zero compact element then either the multiplication in A is trivial or A has a closed ideal invariant under all central multipliers. Proof. Let an element a ∈ A be compact. Then I = {b : La Rb ∈ K(A)} is a nonzero closed ideal in A invariant under central multipliers. So we have to assume that I = A. Setting J = {c : Lc Rb ∈ K(A)} for all b ∈ A, we similarly reduce to the case that J = A. In other words, we may suppose that A is bicompact. Recall that a subspace invariant under an algebra of operators and its commutant is called hyperinvariant for this algebra. Note that the set of all central

96


multipliers is the commutant of Mul(A). So our aim is to show that Mul(A) has a non-trivial hyperinvariant subspace. Since Mul2 (A) is an ideal in Mul(A) it suffices to show the same for Mul2 (A) (see for example [31]). By Corollary 5.9, Mul2 (A) is a radical algebra of compact operators. Hence it has a hyperinvariant subspace by [24]. Is this possible to strengthen the result and to prove the existence of a total chain of closed ideals? We will show that the answer to this question is negative. Recall that a chain (i.e., a set linearly ordered by the inclusion) N of closed subspaces of a Banach space X is total if it is not contained in a larger chain of subspaces. This is equivalent to the conditions that N is complete and for any elements Y1 ⊂ Y2 of N, either dim(L2 /L1 ) = 1 or there exists an intermediate subspace in N. Let us call by a gap in the lattice of closed ideals of a Banach algebra A a pair I1 ⊂ I2 of closed ideals without intermediate ideals, and in this case the quotient I2 /I1 is called a gap-quotient of the lattice. It is easy to show by transfinite induction that if dim(I2 /I1 ) = 1 for any gap, then A has a total chain of closed ideals (moreover, each chain of ideals extends to a total one). An example of a gap is a pair (0, I) where I is a minimal closed ideal. So if each chain of closed ideals in A extends to a total one then each minimal closed ideal is one-dimensional. We show now that these properties can fail in the class of radical bicompact algebras. Then it will be shown that for radical hypofinite algebras the situation is different. Theorem 5.20. (i) There is a radical bicompact Banach algebra without a total chain of closed ideals. (ii) A radical bicompact Banach algebra can have an infinite-dimensional minimal closed ideal. Proof. Let A be a commutative bicompact radical Banach algebra with a topologically irreducible representation π : A −→ B(X) (see Theorem 5.15). On the Banach space B = A⊕X with the norm a⊕x = max{a, x} introduce a multiplication by (a⊕x)(b⊕y) = ab⊕π(a)y. Then B is a Banach algebra. Since n (a⊕x) = an ⊕π(an−1 )x, then (a⊕x)n ≤ (a + x)an−1 for every n > 0, whence B is radical. We show that B is a bicompact algebra. For any a⊕x, b⊕y ∈ B, the operator T = La⊕x Rb⊕y maps any c⊕z into acb⊕π(ac)y. As ball(B) = ball(A)⊕ball(X), we obtain that T (ball(B)) ⊂ La Rb (ball(A))⊕π(La (ball(A))y). As all operators LaRb in A are compact, it suffices to prove the precompactness of the set π(La (ball(A))y). In other words, we have to show that any operator Sy : c −→ π(ac)y is compact. If take y ∈ π(A)X with y = π(d)z for some d ∈ A and z ∈ X, then Sy is compact because it decomposes through La Rd . It follows that Sy is compact for any y in the linear span Y of π(A)X. But Y is dense in X


97

because it is invariant for π(A). Hence for any y ∈ X there is a sequence yn → y in Y . It follows that Sy − Syn → 0, so Sy is compact for every y ∈ X. The subspace I = 0⊕X is a closed ideal of B and it follows easily from topological transitivity of π that I is a minimal closed ideal. Moreover, each nonzero closed ideal J of B contains I. Indeed, J cannot be a subspace of I. Hence there is a⊕x ∈ J with a = 0. But then 0⊕π(b)π(a)y = (b⊕0)(a⊕x)(0⊕y) ∈ J for any b ∈ A, y ∈ X, whence I ⊂ J. We see that B has no total chains of closed ideals and has an infinitedimensional minimal closed ideal. In the remaining part of the section we obtain some “affirmative” results. Lemma 5.21. If A is a radical compact Banach algebra and J ⊂ I is a gap of closed ideals of A then AI ⊂ J or IA ⊂ J. Proof. Assume the contrary. Then AI = IA = I (otherwise we obtain an intermediate ideal) whence AIA = I. One may assume that dim(I/J) > 1 because otherwise the statement is trivial. Let π be the natural representation of Mul(A) on the space X = I/J. It is topologically irreducible because if Y is an invariant closed subspace of π then {x ∈ I : x/J ∈ Y } is a closed ideal between J and I. By Corollary 5.9, the algebra Mul2 (A) is contained in the radical of Mul(A). If π(Mul2 (A)) contains a non-zero compact operator then it has a non-trivial invariant closed subspace by the Lomonosov Theorem [18]. As π(Mul2 (A)) is an ideal of π(Mul(A)), this implies that π(Mul(A)) has a non-trivial invariant closed subspace, a contradiction. As we saw in the proof of Theorem 5.8, the operator La2 Rb is a compact operator in Mul2 (A) for every a, b ∈ A. Therefore, π(La2 Rb ) is also a compact operator. By the above, π(La2 Rb ) = 0. In other words, a2 Ib ⊂ J. Since IA = I, we get that a2 I ⊂ J. Thus π(La )2 = 0 for all a ∈ A, whence A3 I ⊂ J by the Nagata–Higman theorem. Since AI = I, we obtain a contradiction. Theorem 5.22. If A is an infinite-dimensional compact radical Banach algebra then any chain of closed ideals of A extends to an infinite chain of closed ideals of A. Proof. Suppose, to the contrary, that there is a maximal chain 0 = J0 ⊂ J1 ⊂ · · · ⊂ Jn = A of closed ideals of A. Then each pair (Jk−1 , Jk ) is a gap. It follows from Lemma 5.21 that AJk A ⊂ Jk−1 for every k > 0. Hence A2n+1 = 0.

98


It follows that Mul(A) is also a nilpotent algebra. Hence it has no non-trivial topologically irreducible representations. But for any gap (Jk−1 , Jk ) its representation on Xk = Jk /Jk−1 is topologically irreducible and at least one of Xk must be infinite dimensional. We obtained a contradiction. Recall that by A(X) we denote the operator norm closure of the ideal F (X) of all finite rank operators on X. Our aim is to show that if A(X) = K(X) then between A(X) and K(X) there are intermediate closed ideals. Corollary 5.23. (i) If dim(K(X)/A(X)) = n (where n is a finite number or ∞) then K(X) has a chain of n different closed ideals, containing F (X). (ii) Let M and N be closed ideals of B(X) with A(X) ⊂ N M ⊂ K(X). If M 2 is not contained in N then B(X) has a closed ideal between N and M . In particular, if (K(X)/A(X))2 = 0 then there is a closed ideal of B(X) between A(X) and K(X). (iii) If the algebra K(X)/A(X) is not nilpotent then every maximal chain of closed ideals of B(X) between A(X) and K(X) is infinite. Proof. (i) The algebra Q(X) = K(X)/A(X) is radical by Corollary 3.54. If its dimension n is finite then clearly it has a chain of n ideals (since the nilpotent algebra Mul(Q(X)) is triangularizable). In any case it is bicompact, so if n = ∞ then it has an infinite chain of ideals by Theorem 5.22. The preimages of these ideals in K(X) form a chain of ideals of K(X) containing F (X). This proves (i). (ii) Assume, to the contrary, that there are no closed ideals between N and M . As M 2 + N is a closed ideal of B(X) strictly containing N , then M = M2 + N. As {T ∈ M : T M ⊂ N } is a closed ideal of B(X) strictly contained in M , then N = {T ∈ M : T M ⊂ N } and, similarly, N = {T ∈ M : M T ⊂ N }. By (i), there is a closed ideal I of K(X) intermediate between M and N . Set J = M IM + N . Then N ⊂ J ⊂ I M. If J = N then M IM ⊂ N whence, by above, IM ⊂ N and therefore I ⊂ N , a contradiction. Thus N J M. As B(X)JB(X) ⊂ B(X)M IM B(X) + N ⊂ M IM + N = J, we obtained that J is an intermediate closed ideal of B(X) between N and M . Part (ii) is proved. (iii) Assuming that Q(X) is not nilpotent, choose a maximal chain (Iα ) of closed ideals of B (X) between A(X) and K(X). If it is finite, namely A(X) = I0 I1 I2 · · · In = K(X),


99

then Ik2 ⊂ Ik−1 n

for every k > 0 by (ii). Then Q(X)2 = 0, a contradiction.

Example 5.24. To construct an example of a Banach space X for which the algebra K(X)/A(X) is not nilpotent, one can use a remarkable result of Willis [34]. Recall that X is said to have approximation property (AP) (respectively, compact approximation property (CAP)) if for each compact set M ⊂ X and each ε > 0, there is a finite rank (respectively, compact) operator S = S(M, ε) with Sx−x < ε for all x ∈ M . If S always can be chosen in such a way that S ≤ C for some fixed C > 0 then one says that X has bounded approximation property (BAP) (respectively, bounded compact approximation property (BCAP)). It was proved in [34] that there exists a space X which has not AP but has BCAP. Let us show that this is a space we need. Indeed, it follows from BCAP that the algebra K(X) has a bounded approximate identity: to construct it one have to take for the index set the set of all pairs λ = (M, ε) where M is a compact subset of X and ε > 0, and denote by Sλ an operator S = S(M, ε) from the definition of BCAP. In particular, K(X)n is dense in K(X) for each n. Hence if K(X)/A(X) is nilpotent then A(X) = K(X). Therefore A(X) has a bounded approximate identity eλ and one can assume that eλ ∈F(X) for each λ. Let us show that this implies AP (in contradiction with the choice of X). It is easy to see (considering rank one operators) that eλ x → x for each x ∈ X. Now if a compact subset M of X and ε > 0 are given, let us choose a finite ε-net M0 in M and an index μ with eμ x − x < ε for all x ∈ M0 . Then eμ x − x < tε for all x ∈ M , where t = 2 + supλ eλ . Let us denote by A and K the closed operator ideals of approximable and compact operators, respectively. Corollary 5.25. There is an infinite chain of closed operator ideals intermediate between A and K. Proof. Let Z be a Banach space with non-nilpotent K(Z)/A(Z) (see Example 5.24). By Corollary 5.23, between K(Z) and A(Z) there is an infinite chain {Iα } of closed ideals of B(Z). For each pair (X, Y ) of Banach spaces, we denote by Uα (X, Y ) the set of all operators T ∈ K(X, Y ) such that AT B ∈ Iα for all A ∈ B(Y, Z) and B ∈ B(Z, X). It is easy to check that each Uα is a closed operator ideal between A and K, that all Uα are different and that they form a chain. Theorem 5.26. If A is a radical approximable Banach algebra then each gapquotient in the lattice of the closed ideals of A is one-dimensional. Proof. Let J ⊂ I be a gap of closed ideals of A. Then either AI ⊂ J or IA ⊂ J by Lemma 5.21. Suppose that IA ⊂ J. Denote by π the natural representation of Mul(A) on I/J. Then we have that π(RA ) = 0, whence π(E(A)) = π(LA ) and π(Mul(A)) ⊂ π(LA ).

100


The map a −→ π(La ) is a topologically irreducible representation of A on I/J. Since such a representation of A must be trivial by Proposition 5.14, it acts on a one-dimensional space. Corollary 5.27. Every radical hypofinite Banach algebra has a total chain of closed ideals, and each minimal closed ideal in such an algebra is one-dimensional. Let us call a subspace I of a Banach algebra A a quasiideal if AIA ⊂ I. Clearly each ideal is a quasiideal. The converse is true if A has a (non-necessarily bounded) approximate identity. Theorem 5.28. Any bicompact radical Banach algebra has a total chain of closed quasiideals. Proof. Closed quasiideals are invariant subspaces of the radical algebra Mul2 (A) of compact operators. Since such algebras are triangularizable, our statement follows.

6. Spectral subspaces of elementary and multiplication operators In this section we consider invariant subspaces of semicompact multiplication operators, on which the operators are surjective (in particular, eigenspaces with nonzero eigenvalues or spectral subspaces corresponding to clopen subsets of spectra non-containing 0). Our approach will be based (apart of the tensor radical technique) on a study of operators acting in ordered pairs of Banach spaces. In Section 6.4 we improve the results for semicompact elementary operators by another technique to show that such invariant subspaces are contained in the component of every quasi-Banach operator ideal. 6.1. Operators on an ordered pair of Banach spaces Let X , Y be Banach spaces, and Y ⊂ X . Suppose that yX ≤ yY

(6.1)

for all y ∈ Y. We refer to such a subspace Y as a Banach subspace of X and call (Y, X ) an ordered pair of Banach spaces. Denote by B (X ||Y) the space of all operators T ∈ B(X ) such that T Y ⊂ Y. It is non-zero, for instance the identity operator 1X in B(X ) lies in B (X ||Y). Theorem 6.1. Let Y be a Banach subspace of a Banach space X . Then T |Y ∈ B(Y) for any T ∈ B (X ||Y), and B (X ||Y) is a unital Banach subalgebra of B(X ) with respect to the norm T B(X ||Y) = max T B(X ), T |Y B(Y) for any T ∈ B (X ||Y).


101

Proof. We first show that T |Y is a bounded operator on Y. To apply the Closed Graph Theorem, it is sufficient to show that the conditions yn Y → 0 and T yn − uY → 0 as n → ∞ for {yn} ⊂ Y imply u = 0. If these conditions hold, then also yX → 0 and T yn − uX → 0 as n → ∞. As T is bounded on X , then u = 0. As a consequence, · B(X ||Y) is a norm on B (X ||Y). This norm is clearly a unital algebra norm that majorizes · B(X ) on B (X ||Y). To finish the proof, it remains to show that · B(X ||Y) is complete. Let {Tn } be a fundamental sequence in B (X ||Y). Then there are T ∈ B(X ) and S ∈ B(Y) such that Tn − T B(X ) → 0 and Tn |Y − SB(Y) → 0 as n → ∞. Then Tn y − T yX → 0 and Tn y − SyX ≤ Tn y − SyY → 0 as n → ∞, for every y ∈ Y. This shows that Y is invariant for T and T |Y = S. As usual, SB(X ,Y) denotes the operator norm of an operator S in B(X , Y). Proposition 6.2. Let Y be a Banach subspace of a Banach space X . Then B(X , Y) is a Banach algebra with respect to the usual norm · B(X ,Y) . Proof. It is clear that B(X , Y) is a Banach space. If S, T ∈ B(X , Y) then ST xY ≤ SB(X ,Y)T xX ≤ SB(X ,Y)T xY ≤ SB(X ,Y)T B(X ,Y)xX for every x ∈ X , whence B(X , Y) is a Banach algebra.

Proposition 6.3. Let Y be a Banach subspace of a Banach space X . Then B(X , Y) is a Banach ideal of B (X ||Y) with respect to ·B(X ,Y) which is a flexible norm. Proof. The inclusion B(X , Y) ⊂ B (X ||Y) follows by Theorem 6.1. Let S ∈ B(X , Y) and P, T ∈ B (X ||Y). It is clear that P S, ST ∈ B(X , Y) ⊂ B (X ||Y). So B(X , Y) is an ideal of B (X ||Y). As SxY ≤ SB(X ,Y)xX for every x ∈ X , one obtains from (6.1) that SxX ≤ SxY ≤ SB(X ,Y)xX and SyY ≤ SB(X ,Y)yX ≤ SB(X ,Y)yY for every x ∈ X and y ∈ Y. Therefore SB(X ) ≤ max SB(X ) , S|Y B(Y)

!

= SB(X ||Y) ≤ SB(X ,Y).

It follows that P ST xY ≤ P |Y B(Y) ST xY ≤ P |Y B(Y) SB(X ,Y) T xX ≤ P |Y B(Y) SB(X ,Y) T B(X ) xX ≤ P B(X ||Y) SB(X ,Y) T B(X ||Y) xX . for every x ∈ X , whence P ST B(X ,Y) ≤ P B(X ||Y) SB(X ,Y) T B(X ||Y) .

102


Theorem 6.4. Let Y be a Banach subspace of a Banach space X . Then every operator T ∈ B (X ) such that T X ⊂ Y belongs to B (X , Y). Proof. By the Closed Graph Theorem, it suffices to show that u = 0 if xn X → 0 and T xn − uY → 0 as n → ∞. Indeed, the last implies that T xn − uX → 0 as n → ∞. As T is bounded on X , then u = 0. 6.2. Invariant subspaces for operators on an ordered pair of Banach spaces We consider those invariant subspaces of an operator on an ordered pair (Y, X ) of Banach spaces on which the operator is surjective. Clearly such a subspace is contained in Y if the operator belongs to B (X , Y). We show that the same is true if the operator belongs to B (X ||Y) and is quasinilpotent modulo B (X , Y). Theorem 6.5. Let X be a Banach space, Y a Banach subspace of X , and let T ∈ B (X ||Y). Assume that (6.2) T ∈ QB(X ,Y) (B (X ||Y)) . If Z is a closed subspace of X such that Z = T Z, then Z ⊂ Y. Moreover, the norms · X and · Y are equivalent on Z, so Z is also closed in Y. Proof. By the Open Mapping Theorem, there is t > 0 such that for each z ∈ Z there is w ∈ Z with T w = z and wX ≤ tzX . −1

Let ε > 0 be such that ε < t . It follows from Proposition 6.3 that B(X , Y) is an ideal of B (X ||Y). By our assumption and Proposition 2.1(iii), there is m ∈ N such that distB(X ||Y) (T m , B(X , Y)) < εm . Therefore there is an operator S ∈ B(X , Y) and an operator P ∈ B (X ||Y) such that ! T m = S + P with max P B(X ) , P |Y B(Y) < εm . Then P yY ≤ εm yY

(6.3)

for every y ∈ Y. It follows from the definition of t that for every z ∈ Z, there is z ◦ ∈ Z with m ◦ T z = z and z ◦ X < tm zX . Let z0 := z ∈ Z be arbitrary. Set z1 = z0◦ , z2 = z1◦ , and so on: zk+1 = zk◦ for every integer k > 0. Thus zk = T m zk+1 = Szk+1 + P zk+1 with zk X ≤ tmk zX . Rewriting this in the form zk − P zk+1 = Szk+1 ,

(6.4)


103

multiplying both sides of the equation by P k and summing obtained equalities for k = 0, 1, 2, . . ., one formally obtains that z = Sz1 + P Sz2 + P 2 Sz3 + · · · . Since SX ⊂ Y, all elements Szk and P one obtains that

k−1

Szk belong to Y. As zk X < t

Szk Y < SB(X ,Y) zk X ≤ tmk SB(X ,Y) zX .

(6.5) mk

zX , (6.6)

It follows from (6.3) and (6.6) that k−1 P Szk Y ≤ εm(k−1) Szk Y ≤ (εt)mk ε−m SB(X ,Y) zX . As εt < 1, we have that ∞ k−1 P Szk Y ≤ k=1

tm S B(X ,Y) zX < ∞. 1 − εm tm

(6.7)

Since · Y is a complete norm on Y, it follows from (6.7) and (6.5) that z ∈ Y with the estimation tm zY ≤ S B(X ,Y) zX . 1 − ε m tm Therefore ·X and ·Y are equivalent on Z. As Z is closed with respect to ·X , it is closed with respect to ·Y . Corollary 6.6. Let X , Y and T be as in Theorem 6.5. Then (i) If λ = 0 is an eigenvalue of T then the eigenspace {x ∈ X : T x = λx} is contained in Y. (ii) If σ0 is a clopen subset of the spectrum σB(X ) (T ) of T and 0 ∈ / σ0 then the spectral subspace Eσ0 (T ) is contained in Y. Proof. Indeed, these subspaces are closed in X , invariant under T , and the restriction of T to everyone of them is invertible. 6.3. Semicompact multiplication operators In this section we apply Theorem 6.5 to semicompact multiplication operators considering their action on an ordered pair of spaces of nuclear and, respectively, bounded operators. 6.3.1. Multiplication operators on an ordered pair of operator ideals. First we estimate the norms of multiplication operators on an ordered pair of components of Banach operator ideals. Let V = V (X, Y ) and U = U (X, Y ), where V and U are Banach operator ideals. We assume that V ⊂ U and that V is a Banach subspace of U . As V is an invariant subspace for the algebra B∗ (U ) of all multiplication operators on U , then the algebra B (X ||Y) contains B∗ (U ) by Theorem 6.1. Lemma 6.7. Let V = V (X, Y ) and U = U (X, Y ) for Banach operator ideals V and U, and V ⊂ U . Then T |V B∗ (V ) ≤ T B∗ (U ) for every T ∈ B∗ (U ).

104


(X)op for brevity. Recall that the norms ·B in Proof. Let W = B (Y ) ⊗B ∗ (V ) B∗ (V ) and ·B∗ (U ) in B∗ (U ) are the quotient norms inherited from W/ ker ψ and W/ ker ϕ respectively, where ψ : W −→ B (V ) and ϕ : W −→ B (U ) are bounded homomorphisms that associate with every a ⊗ b ∈ W the operator La Rb . Let P = ϕ (w) and S = ψ (w) for some w ∈ W . If ϕ (w) = 0 then P x = 0 for every x ∈ U . In particular, P x = 0 for every x ∈ V and ψ (w) = S = P |V = 0. This shows that ker ϕ ⊂ ker ψ, and we are done.

Proposition 6.8. Let V = V (X, Y ) and U = U (X, Y ) for Banach operator ideals V and U, and V ⊂ U . Then T B(U||V ) ≤ T B∗ (U ) for every T ∈ B∗ (U ). Proof. Indeed, as ·B∗ (U ) majorizes the norm ·B(U) on B∗ (U ), we obtain from Lemma 6.7 that ! T B(U ||V ) = max T B(U) , T |V B(V ) ≤ T B∗ (U) for every T ∈ B∗ (U ).

6.3.2. Applications to semicompact multiplication operators. Let X, Y be arbitrary Banach spaces. Let X = B (X, Y ) and Y = N (X, Y ), the space of all nuclear operators X −→ Y . It is clear that Y is a Banach subspace of X . Also, X and Y are Banach operator bimodules over the algebras B(X) and B(Y ), so the algebra B (X ||Y) contains the algebra B∗ (X ) of all multiplication operators T = Lai Rbi with ai bi < ∞ i

i

where ai ∈ B (Y ), bi ∈ B (X) . Recall that a multiplication operator T is called semicompact if it can be written in the form Lai Rti + Lsj Rbj , (6.8) T = i

j

where all ai and bj are compact operators, and ai ti + sj bj < ∞. i

j

Also, an elementary operator T is called semifinite if it can be written in the form (6.8) with ai and bj of finite rank. The algebras of all semicompact multiplication 1 (X ) operators on X and all semifinite elementary operators on X are denoted by K 2 and F 12 (X ), respectively. 1 (X ) ⊂ B (X ||Y). In particular, from above we have that K 2

Theorem 6.9. Let X = B (X, Y ) and Y = N (X, Y ). Then 1 (X ) ⊂ QB(X ,Y) (B (X ||Y)) . K 2

(6.9)


105

Proof. By Corollary 4.9, we have that

1 (X ) ⊂ QF (X ) B∗ (X ) . K 1 2

(6.10)

2

As ·B(X ||Y) ≤ ·B∗ (X ) on B∗ (X ) by Proposition 6.8, it follows that QF 1 (X ) B∗ (X ) ⊂ QF 1 (X ) (B (X ||Y)) . 2

(6.11)

2

Since SX ⊂ Y for every S ∈ F 12 (X ), we have that F 12 (X ) ⊂ B(X , Y) by Theorem 6.4. Therefore, we obtain that QF 1 (X ) (B (X ||Y)) ⊂ QB(X ,Y) (B (X ||Y)) ,

(6.12)

2

and (6.9) follows from (6.10), (6.11) and (6.12).

Now we are able to apply Theorem 6.5 to obtain the following Theorem 6.10. Let T be a semicompact multiplication operator on B(X, Y ). Suppose that a closed subspace Z of B(X, Y ) is invariant for T and that T is surjective on Z. Then Z consists of nuclear operators, and the usual operator norm is equivalent to the nuclear norm on Z. In particular, all eigenspaces of T corresponding to non-zero eigenvalues and all spectral subspaces of T corresponding to clopen subsets of σ (T ) non-containing 0 consist of nuclear operators. The following result holds for integral semicompact operators by Proposition 4.14, Theorems 4.15 and 6.10. Theorem 6.11. Let Ta,b,s,t be an integral semicompact operator on X = B(X, Y ) in the conditions of Proposition 4.14 or Theorem 4.15. Then all invariant subspaces of Ta,b,s,t on which it is surjective consist of nuclear operators. In particular, each solution x of the equation Ta,b,s,t x = λx where λ = 0, is a nuclear operator. We may apply previous results to matrix multiplication operators (see Section 4.3.2). Corollary 6.12. Let a matrix (Tpq )np,q=1 consist of semicompact multiplication operators and let T be the matrix multiplication operator defined by this matrix. Then the spectral subspaces of T that correspond to clopen subsets of σ(T ) non-containing 0, consist of n-tuples of nuclear operators. Proof. Let X = B(X, Y )(n) , the direct sum of n copies of B(X, Y ), Y = N (X, Y )(n) and U = B(X, Y ). Then it is easy to see that 1 (U ) ⊂ Mn B∗ (U ) ⊂ B (X ||Y) and Mn F 1 (U ) ⊂ B(X , Y). Mn K 2 2

106


Now a similar argument as in Theorem 6.9 shows that 1 (U )) ⊂ QB(X ,Y) (B (X ||Y)) , Mn (K 2

and it remains to apply Theorem 6.5.

6.4. Semicompact elementary operators Let X, Y be Banach spaces. Assume now that T is an elementary operator on B(X, Y ): n k T = Lai Rxi + Lyj Rbj , i=1

j=1

where all xi ∈ B (X), yj ∈ B(Y ), ai ∈ K(Y ), bj ∈ K(X). According to our terminology, T is a semicompact elementary operator. Our aim is to show that the statement of Theorem 6.10 in this case can be considerably strengthened: invariant subspaces on which T is surjective are contained in the component J (X, Y ) of each quasi-Banach operator ideal J. In this situation the approach based on the tensor products of Banach algebras and the tensor spectral radius theory is not directly applicable and for the proof that some power of T is close (in a proper sense) to a semifinite elementary operator, we use the arguments based on the analysis of triangularizable sets of compact operators. 6.4.1. Quasi-Banach operator ideals. Recall that a quasinorm on a linear space L is a map ·L : L → R+ satisfying the conditions x + yL ≤ tL (xL + yL ) for all x, y ∈ L and some tL ≥ 1,

(6.13)

λxL = |λ| xL for all λ ∈ C, x ∈ L, and xL = 0 iff x = 0. By [17, Page 162], each quasinorm generates a linear (metrizable) Hausdorff topology on L. We say that L is complete under the quasinorm if it is complete in this topology. Furthermore, a quasi-Banach operator ideal J (see [20]) consists of components J (X, Y ) ⊂ B (X, Y ) complete under a quasinorm ·J(X,Y ) = ·J , where X and Y run over Banach spaces, and satisfying the following conditions 1) tJ(X,Y ) = tJ for some tJ ≥ 1 and all Banach spaces X and Y , where tJ(X,Y ) is the constant tL in (6.13) for L = J (X, Y ). 2) axbJ ≤ a xJ b for all x ∈ J (X, Y ), a ∈ B (Y, Z) , b ∈ B(W, X), where Z and W run over Banach spaces, 3) xJ = ||x|| for each operator x of rank one. By [20, Theorem 6.2.5], each quasi-Banach ideal J has an equivalent quasinorm |·|J with the property that there is a number p such that 0 < p ≤ 1 and p

p

p

|x + y|J ≤ |x|J + |y|J

(6.14)


107

for every x, y ∈ J (X, Y ) and for all Banach spaces X, Y (one can take p as a number p satisfying (2t) = 2 for t ≥ tJ ). We assume that a quasinorm in consideration satisfies this condition, and write · p or · p,J instead of |·|J . In this case we say that J is a p-Banach operator ideal. It should be noted that the topology of J (X, Y ) is given by the metric d (x, y) = x − ypp . In the same way we denote the corresponding quasinorm on bounded operators T on J (X, Y ): T p = T p,J = inf {t > 0 : T xp ≤ txp for all x ∈ J (X, Y )} . Lemma 6.13. Let J be a p-Banach operator ideal, and let T be an elementary n operator on B (X, Y ), T x = i=1 ai xbi for every x ∈ B (X, Y ), where ai ∈ B (Y ), bi ∈ B (X). Then T is bounded on J (X, Y ) and n 1−p ai bi xp T xp ≤ n p i=1

for all x ∈ J (X, Y ). Proof. It follows from (6.14) under |·|J = · p that T xpp ≤

n

ai p bi pxpp

i=1

for all x ∈ J (X, Y ). Since the function f (t) = tp is concave for t ≥ 0 and 0 < p ≤ 1, we obtain that n p n p ti ≤ n1−p ti . (6.15) i=1

i=1

Applying this to ti = ai bi , we get that p n p 1−p ai bi xpp T xp ≤ n i=1

which gives what we need.

In a short form the statement of the previous lemma can be written as follows: If T =

n

Lai Rbi then T p ≤ n

1−p p

n

i=1

ai bi .

i=1

Similarly we obtain the following Lemma 6.14. Let a be a finite rank operator in B (X, Y ). Then ap ≤ n

1−p p

aN (X,Y ) ,

where n is the rank of a and ·p is the p-norm of a p-Banach operator ideal J.

108


Proof. It is easy to check that for any ε > 0 there are rank one operators ai such that n a= ai , i=1

where n is the rank of a, and n

ai ≤ aN (X,Y ) + ε.

i=1

On the other hand, app

≤

n i=1

p ai p

=

n i=1

p

ai ≤ n

1−p

n

p ai

p ≤ n1−p aN (X,Y ) + ε

i=1

by (6.15). As ε is arbitrary, we obtain that ap ≤ n

1−p p

aN (X,Y ) .

6.4.2. Quasinilpotence of semicompact elementary operators modulo semifinite ones with respect to a quasinorm. If W is a closed subspace of a Banach space X then for each x ∈ X, we will write x/W instead of x + W for the corresponding element of X/W . If moreover a is an operator on X leaving W invariant then we denote by a|W and a|X/W its restriction to W and, respectively, the operator induced by a in X/W . Lemma 6.15. Let W be a closed subspace of a Banach space X, and let a, b be operators on X which preserve W invariant. Then ab ≤ 2a|W b + ab|X/W .

(6.16)

Proof. For any x ∈ X and ε > 0, choose y ∈ W with bx−y ≤ (bx) /W X/W +ε. Then y ≤ bx + (bx) /W X/W + ε ≤ 2bx + ε, whence we obtain that abx ≤ ay + abx − y ≤ a|W y + a( (bx) /W X/W + ε) ≤ a|W (2bx + ε) + a(b|X/W x/W X/W + ε). Since x/W X/W ≤ x and ε is arbitrary, we obtain that abx ≤ (2a|W b + ab|X/W )x which is what we need.

Lemma 6.16. Let 0 = X0 ⊂ X1 ⊂ X2 ⊂ · · · ⊂ Xk ⊂ X be a chain of closed subspaces in a Banach space X. Let m ≥ k and let a1 , . . . , am ∈ B(X) preserve all Xj invariant: ai Xj ⊂ Xj . If ai ≤ α for all i, and ai |Xj /Xj−1 ≤ β for all i, j, then k k m−k α β . a1 a2 · · · am ≤ 2m Cm


109

Proof. We use induction in m, k. For the base of the induction, note that the statement is evidently true for k = 0 and for m = k, and Lemma 6.15 establishes it for k = 1, m = 2. Now assuming that the statement holds for (m − 1, k − 1) and (m − 1, k) we prove that it holds for (m, k). Indeed, setting W = X1 , a = a1 , b = a2 · · · am in the notation of Lemma 6.15, we obtain from (6.16) that a1 a2 · · · am ≤ 2a1 |X1 a2 · · · am + a1 a2 · · · am |X/X1 . By the induction assumption, we have that k αk β m−1−k . a2 · · · am ≤ 2m−1 Cm−1

Furthermore, the operators ai |X/X1 preserve the chain {Xi /X1 : i ≤ k} which consists of k − 1 non-trivial elements. Hence again by the induction assumption, we obtain that k−1 k−1 m−k α β . a2 · · · am |X/X1 ≤ 2m−1 Cm−1

Therefore k−1 k−1 m−k k a1 a2 · · · am ≤ 2β2m−1 Cm−1 αk β m−1−k + α2m−1 Cm−1 α β k−1 k k k m−k ≤ 2m αk β m−k (Cm−1 + Cm−1 ) = 2m Cm α β .

Lemma 6.17. Let K be a finite set of compact operators in the radical of an operator algebra A ⊂ B(X), and let F be a bounded subset of A. Let λ ∈ (0, 1). For each λ (m) denote the set of all products b1 . . . bm of elements in K ∪ F in m, let EK,F which the number of those bi that belong to K is greater than or equal to λm. Then λ EK,F (m)1/m → 0 for m → ∞. Proof. Without loss of generality, one may assume that K ∪ F = 1. By [24] (see also [25]), for each ε > 0 there is a finite chain 0 ⊂ X1 ⊂ · · · ⊂ Xk ⊂ X of invariant subspaces for A such that b|Xj /Xj−1 ≤ ε for all b ∈ K and all j ≤ k. It follows that c|Xj /Xj−1 ≤ ε if c = b1 . . . bp a, where bi ∈ F and a ∈ K. λ Each product b1 . . . bm ∈ EK,F (m) can be written in the form c1 c2 . . . cl , where all of ci are as above and l ≥ λm. Applying the result of Lemma 6.16, we obtain that b1 . . . bm ≤ 2l Clk εl ≤ 2m mk ελm . Thus λ (m)1/m ≤ 2mk/m ελ ≤ 3ελ EK,F

for sufficiently big m. Since ε is arbitrary, we are done.

A subset M of a Banach algebra A is called bicompact if La Rb is a compact operator for every a, b ∈ M .

110


Proposition 6.18. Let M be a finite bicompact subset of A in the radical of a Banach algebra A, and let N be a bounded subset of A. For each m, let H(m) denote the set of all products x1 · · · xm of elements in M ∪ N in which the number of those xi that belong to M is greater than or equal to m/2. Then H(m)1/m → 0 under m → ∞. Proof. We may assume that A is unital and 1 is the unit of A. Let K be the set of all operators La Rb on A, where a, b ∈ M . Let F = {La : a ∈ N } ∪ {Ra : a ∈ N }. We claim that every product w in H(m) can be written as T (1), where T is a product of operators in which the number of operators in K is greater than or equal to [m/4] (and the number of operators in F is less than or equal to m/2 + 1). Indeed, we do as follows. Represent w as the product of w1 and w2 in which of each the number of those xi that belong to M is greater than or equal to [m/4]. Let a0 = 1, w1 = w3 xi v1 and w2 = v2 xj w4 for some xi , xj ∈ M , where v1 and v2 do not contain any elements from M as a factor. Then w = w3 a1 w4 , where a1 = S1 Lv1 Rv2 (a0 ) and S1 = Lxi Rxj ∈ K. Arguing by induction, we obtain that w = w2k+1 ak w2k+2 , where ak = Sk Pk (ak−1 ), Sk ∈ K and Pk is a product of operators in F , for k ≤ [m/4]. So we obtain the required representation w as T (1) for some k ≥ [m/4]. Now it follows in the notation of Lemma 6.17 that 1/3 2m H(m) ≤ EK,F 3 for sufficiently big m, and it remains to apply Lemma 6.17. k n Theorem 6.19. Let T = i=1 Lai Rxi + j=1 Lyj Rbj be a semicompact elementary operator on B(X, Y ), where all xi ∈ B (X), yj ∈ B(Y ), ai ∈ K(Y ), bj ∈ K(X). (n+k)m Lci Rdi such Then for any ε > 0, there is m ∈ N and an operator S = i=1 that T m − Sp < εm and ci or di is of finite rank for each i, where ·p is the p-norm of a p-Banach operator ideal J. Proof. A required decomposition of the operator T m into the sum of (n + k)m summands can be written as T m = T 1 + T2 , where in T1 we gather those summands where the number of factors Lai is more than the number of factors Lyj (hence their number ≥ m/2), while summands in T2 have more factors Rbj than factors Rxi . Let A = B(Y )/A(Y ) and q : B(Y ) −→ A be the standard epimorphism. Let M = {q(a1 ), . . . , q(an )} and N = {q(y1 ), . . . , q(yk )}. Then M is a bicompact subset of A in the radical of A. Writing T1 as Lwi Rzi , where Lwi Rzi are the above summands of T1 in the decomposition of T m , we note that the corresponding family H(m) (see the above lemma) consists of all products of elements which are


111

q-images of left coefficients wi of summands in T1 . By Lemma 6.18, there is m such that H(m) < εm . This means that for every wi there is a finite rank operator ri with ri − wi < εm .

Then, setting S1 = Lrs Rzs , we obtain a semifinite multiplication operator such that S1 − T1 can be represented in the form Lvi Rzi with vi zi < (n + k)m εm . i

For brevity, we rewrite this in the form S1 − T1 ∗ < (n + k)m εm . Similarly, we find a semifinite operator S2 = Les Rfs , where all fs are finite rank operators, with S2 − T2 ∗ < (n + k)m εm . Hence setting S = S1 + S2 we obtain that S is semifinite and T − S∗ < 2(n + k)m εm . As the number of elementary summands (of length one) in S is (n + k)m by our choice then we obtain that T m − Sp < ((n + k)m )

1−p p

2(n + k)m εm .

by Lemma 6.13. Changing ε by γε for sufficiently small γ, we obtain the required inequality. 6.4.3. Spectral subspaces of semicompact elementary operators. Now we are able to prove the following Theorem 6.20. Let T = ni=1 Lai Rxi + kj=1 Lyj Rbj be a semicompact elementary operator on B(X, Y ), where all xi ∈ B (X), yj ∈ B(Y ), ai ∈ K(Y ), bj ∈ K(X). Suppose that T Z = Z for a closed subspace Z of B(X, Y ). Then Z is contained in J (X, Y ) for any quasi-Banach operator ideal J. Proof. One may suppose that J is a p-Banach operator ideal with p-norm ·p for 0 < p ≤ 1. By the Open Mapping Theorem, there is t > 0 such that for each z ∈ Z there is w ∈ Z with T w = z and w ≤ tz. Take ε > 0 such that ε < t−1 , and choose m and S as in Theorem 6.19. Setting P = T m − S, we have that P p ≤ εm . On the other hand, S maps each operator from B (X, Y ) into an operator of rank ≤ d, where d is the sum of the ranks of finite rank coefficients of S. As J contains all finite rank operators, we have that Sx ∈ J (X, Y ) and Sxp ≤ d

1−p p

SxN (X,Y )

112


by Lemma 6.14, for every x ∈ B(X, Y ). It follows from Theorem 6.4 that S is bounded as an operator B (X, Y ) −→ N (X, Y ). Let s be its norm as such an operator. Then SxN (X,Y ) ≤ s x for every x ∈ B(X, Y ). As a result, we obtain that Sxp ≤ d

1−p p

s x

(6.17)

for all x ∈ B(X, Y ). Now we may argue as in the proof of Theorem 6.5. Then, as we saw, each z ∈ Z can be expressed as in (6.5): z=

∞

P j Szj+1

j=0

with the estimation zj ≤ tmj z

(6.18)

for every j (see (6.4) in Theorem 6.5). As all P j Szj+1 ∈ J (X, Y ), we may estimate their p-norms P j Szj+1 p as follows. We have that P j Szj+1 p ≤ P jpSzj+1 p and that Szj+1 p ≤ d

1−p p

szj+1 ≤ d

1−p p

stm(j+1) z

by (6.17) and (6.18). By our choice, we have that P p ≤ εm . So we obtain that 1−p 1−p P j Szj+1 p ≤ εmj d p stm(j+1) z = d p stm z (εt)mj , whence ∞

∞ 1−p p j P j Szj+1 pp ≤ d p stm z ((εt)mp ) < ∞

j=0

j=0

because of (εt) < 1. As J (X, Y ) is complete under ·p , the convergence of this series implies that z ∈ J (X, Y ). mp

As a consequence, we obtain the following Corollary 6.21. Let T be a semicompact elementary operator on B (X, Y ). Then all eigenspaces of T corresponding to non-zero eigenvalues and all spectral subspaces of T corresponding to clopen subsets of σ (T ) non-containing 0 are contained in J (X, Y ) for any quasi-Banach operator ideal J.


113

References [1] J.C. Alexander, Compact Banach algebras, Proc. London Math. Soc. (3) 18 (1968), 1–18. [2] B. Aupetit, Propri´ etés spectrales des algèbres de Banach, Lect. Notes Math., 735, Springer-Verlag, Berlin, 1979. [3] B. Aupetit, M. Mathieu, The continuity of Lie homomorphisms, Studia Math. 138 (2) (2000), 193–199. [4] B.A. Barnes, Banach algebras which are ideals of a Banach algebra, Pacific J. Math. 38, 1 (1971), 1–7. [5] B.A. Barnes, Density theorems for algebras of operators and annihilator Banach algebras, Mich. Math. J. 19 (1972), 149–155. [6] J. Bergh and J. L¨ ofstr¨ om, Interpolation spaces, An introduction, Springer-Verlag, Berlin, 1976. [7] M.A. Berger and Y. Wang, Bounded semigroups of matrices, Linear algebra Appl. 166 (1992), 21–27. [8] F.F. Bonsall, Operators that act compactly on an algebra of operators, Bull. London Math. Soc. 1 (2) (1969), 163–170. [9] F.F. Bonsall, J. Duncan, Complete normed algebras, Springer-Verlag, Berlin, Heidelberg, New York, 1973. [10] N. Bourbaki, Elements of mathematics, Algebra 1, Chapters 1–3, Springer-Verlag, Berlin, 1998. [11] R.E. Curto, Spectral theory of elementary operators, in “Elementary operators and applications”, Editor M. Mathieu, World Sci. Publ., Singapore, New Jersey, London, 1992, 3–54. [12] A. Defant, K. Floret, Tensor norms and operator ideals, Elsevier, Amsterdam, 1993. [13] P.G. Dixon, Topologically irreducible representations and radicals in Banach algebras, Proc. London Math. Soc. (3) 74 (1997), 174–200. [14] C.-K. Fong, H. Radjavi, On ideals and Lie ideals of compact operators, Math. Ann. 262 (1983), 23–28. [15] E. Hille, R. Phillips, Functional analysis and semigroups, AMS, Providence, 1957. [16] M. Kennedy, V.S. Shulman, Yu.V. Turovskii, Invariant subspaces of subgraded Lie algebras of compact operators, Integr. Eq. Oper. Theory 63 (2009), 47–93. [17] G. K¨ othe, Topological vector spaces I, Springer-Verlag, New York, 1969. [18] V.I. Lomonosov, Invariant subspaces for operators commuting with compact operators, Funct. Anal. Appl. 7 (1973), 213–214. [19] C.J. Read, Quasinilpotent operators and the invariant subspace problem, J. London Math. Soc. (2) 56 (1997), 595–606. [20] A. Pietsch, Operator ideals, VEB Deutscher Verlag der Wissenschaften, Berlin, 1978. [21] G.-C. Rota and W.G. Strang, A note on the joint spectral radius, Indag. Math. 22 (1960) 379–381. [22] W. Rudin, Functional analysis, McGraw-Hill, New York, 1991. [23] A.F. Ruston, Fredholm theory in Banach spaces, Cambridge Univ. Press, Cambridge, 1986.

114


[24] V.S. Shulman, On invariant subspaces of Volterra operators, Functional Anal. i Prilozen. 18 (2) (1984), 84–85 (in Russian). [25] V.S. Shulman, Yu.V. Turovskii, Joint spectral radius, operator semigroups and a problem of W. Wojty´ nski, J. Funct. Anal. 177 (2000), 383–441. [26] V.S. Shulman, Yu.V. Turovskii, Radicals in Banach algebras and some problems in the theory of radical Banach algebras, Funct. Anal. Appl. 35 (2001), 312–314. [27] V.S. Shulman, Yu.V. Turovskii, Formulae of joint spectral radii for sets of operators, Studia Math. 149 (2002), 23–37. [28] V.S. Shulman, Yu.V. Turovskii, Invariant subspaces of operator Lie algebras and Lie algebras with compact adjoint action, J. Funct. Anal. 223 (2005), 425–508. [29] V.S. Shulman, Yu.V. Turovskii, Topological radicals, I. Basic properties, tensor products and joint quasinilpotence, Banach Center Publ., 67 (2005), 293–333. [30] V.S. Shulman, Yu.V. Turovskii, Application of topological radicals to calculation of joint spectral radii, preprint: arXiv:0805.0209 (2 May 2008). [31] Yu.V. Turovskii, Volterra semigroups have invariant subspaces, J. Funct. Anal. 162 (2) (1999), 313–323. [32] K. Vala, On compact sets of compact operators, Ann. Acad. Sci. Fenn. Ser. A I 351 (1964), 1–8. [33] E. Vesentini, On the subharmonicity of the spectral radius, Boll. Un. Mat. Ital. 4 (1968), 427–429. [34] G. Willis, Compact approximation property does not imply approximation property, Studia Math. 103 (1992), 99–108. [35] W. Wojtynski, A note on compact Banach-Lie algebras of Volterra type, Bull. Acad. Polon. Sci., Ser. sci. math., astr. et phys. 26 (1978), 2, 105–107. [36] W. Wojtynski, On the existence of closed two-sided ideals in radical Banach algebras with compact elements, Bull. Acad. Polon. Sci., Ser. sci. math., astr. et phys. 26 (1978), 2, 109–113. [37] W. Wojtynski, Quasinilpotent Banach-Lie algebras are Baker-Campbell-Hausdorff, J. Funct. Anal. 153 (1998), 405–413. [38] E.I. Zelmanov, On Engel Lie algebras, DAN USSR, 292 (1987) no. 2, 265–268 (in Russian). V.S. Shulman Department of Mathematics, Vologda State Technical University, 15 Lenina str. Vologda 160000, Russian Federation e-mail: shulman [email protected] Yu.V. Turovskii Institute of Mathematics and Mechanics, National Academy of Sciences of Azerbaijan, 9 F. Agayev Street Baku AZ1141, Azerbaijan e-mail: [email protected]

An Elementary Approach to Elementary Operators on B(H) V.S. Shulman and L. Turowska Abstract. We review some properties of elementary operators on the space of bounded operators on a Hilbert space H which they share with operators on H. Mathematics Subject Classification (2000). Primary 47B47; Secondary 47A62, 47L05, 47B10 . Keywords. Elementary operator, normal, derivation.

1. Introduction In 1975 J. Anderson and F. Foias wrote a paper [2] whose title could be used (with modifications) in many later papers on elementary operators. In the present paper we discuss properties which operators on a Hilbert space H share with elementary operators on B(H). The reason of the search of common properties is that an ˜ = n La∗ Rb∗ elementary operator Δ = nk=1 Lak Rbk has a formal adjoint Δ k=1 k k which turns into a proper adjoint if restricted to the ideal S2 of Hilbert-Schmidt operators, and so it is natural to expect that their adjointness on B(H) is not absolutely formal. After discussion of the general case we come to normal elementary operators (that is those whose coefficient families are commutative and consists of normal operators) and study which properties of normal operators on Hilbert space they share. Our paper is a kind of review but some formulations of the results seem to be new. Several statements of the paper can be extended to a more wide class of multiplication operators, but we prefer to restrict ourselves to elementary operators which allows us to avoid more complicated technique of Varopoulos algebras, Haagerup tensor products, spectral synthesis and so on. Moreover we hope that such approach makes the subject really elementary and gives the possibility to present the results with ideas of their proofs in a text of reasonable length. The The second author was supported by the Swedish Research Council.


115

116

V.S. Shulman and L. Turowska

reader can find transparent proofs and various extensions of the main part of our results in [19, 22, 23]. 1.1. Notations Let H be a Hilbert space, B(H) be the space of bounded linear operators on H. We denote by Sp , 1 ≤ p < ∞, a Schatten-von Neumann ideal and write || · ||p for the corresponding norm and let S∞ to denote the space of compact operators. Let us say that an operator (or a family of operators) is multicyclic or has finite multicyclicity if there is a finite set of vectors which is not contained in a proper closed invariant subspace. For a subset M of a metric space we say that its Hausdorff dimension does not exceed a number r > 0 if there exists C > 0 such that for > 0 there is a covering B = {βj } of M by pairwise disjoint Borel sets with diamβj < and |B|r := ( j (diamβj )r ≤ C. If A = (A1 , . . . , An ) is a family of commuting normal operators then by σ(A) we denote the joint spectrum, and by EA (·) the spectral measure of A. We say that the essential dimension of A does not exceed r > 0 (and write ess-dim A ≤ r) if there is a subset D of σ(A) such that EA (σ(A) \ D) = 0 and dim(D) ≤ r.

2. Approximate inverse intertwinings The proofs of many further statements are based on several results of very general nature which we gather in the present section. The reader can consider them as exercises in functional analysis, (s)he can also find their solutions in Section 6 of [22]. Let X and Y be topological vector spaces, Φ : X → Y a continuous imbedding with dense range, and let S and T be operators acting in X and Y, respectively, intertwined by the mapping Φ: T Φ = ΦS. We write in this case that we are given an intertwining triple (or just an intertwining) (Φ, S, T ). A net of linear mappings Fα : Y → X is called an approximate inverse intertwining (AII) for the intertwining (Φ, S, T ) if (a) Fα Φ → 1X , (b) ΦFα → 1Y and (c) Fα T − SFα → 0X in the topology of simple convergence. Denote by Φ−1 the full inverse image under the mapping Φ: Φ−1 (M ) = {x ∈ X | Φ(x) ∈ M } for any M ⊂ Y (non-necessarily M ⊂ Φ(X)). As usual the range of a map X is denoted by im X. Theorem 2.1. If the intertwinings (Φ, Si , Ti ), 1 ≤ i ≤ n, have a common AII, then −1 im Ti ⊂ im Si . Φ i

i

An Elementary Approach to Elementary Operators on B(H)

117

Let H be a Hilbert space equipped with the weak operator topology. Corollary 2.2. If X = H and (Φ, S, T ) has an AII, then Φ(ker S ∗ ) ∩ im T = {0}. Theorem 2.3. Let Φ intertwine pairs Si , Ti (i = 1, 2). Suppose that X is a Banach space equipped with a weak topology, and ||S2 x|| ≤ ||S1 x|| for any x ∈ X. If (Φ, S1 , T1 ) has AII then T1−1 (im Φ) ⊂ T2−1(im Φ) and ||Φ−1 T2 y|| ≤ ||Φ−1 T1 y|| for any y ∈

(2.1)

T1−1(im Φ).

The following result is an immediate consequence of Theorem 2.3. Corollary 2.4. Let X = H. Suppose that S is a normal operator on H and that the intertwinings (Φ, S, T1 ), (Φ, S ∗ , T2 ) have AII’s (not necessarily coinciding). Then T1−1 (im Φ) = T2−1 (im Φ) and ||Φ−1 T2 y|| = ||Φ−1 T1 y|| for any y ∈ T1−1(im Φ) = T2−1 (im Φ). In particular, ker T1 = ker T2 . Let (Φ, S, T ) be an intertwining. If X is a dual Banach space with the weak-∗ topology (for example if X = H) then to obtain an AII it suffices to construct a net of operators Fα : Y → X which satisfies a weakened version of (AII)-conditions: (a ) Fα Φx is bounded for each x ∈ X, (b ) ΦFα → 1Y and (c ) (Fα T − SFα )y is bounded for each y ∈ Y . In general a net satisfying ((a ), (b ), (c )) is called an approximate inner semiintertwining for (Φ, S, T ) (AIS, for short). Denote by X∗ the space of continuous antilinear functionals on X, endowed with the weak-* topology (in particular, H∗ = H). The adjoint operators (on X ∗ or between X ∗ and Y ∗ ) are defined in the usual way. In particular, the adjoint of an operator on H has the usual meaning. It is not difficult to see that if {Fα } is an AII for (Φ, S, T ) then {Fα∗ } is an AII for (Φ∗ , T ∗ , S ∗ ). Let Φ : H → Y intertwine operators S, S ∗ with T1 , T2 . Let {Fα } : Y → H be an AII for the intertwining (Φ, S, T1 ). It is called a ∗-approximate inverse intertwining (∗-AII) for the ordered pair ((Φ, S, T1 ), (Φ, S ∗ , T2 )) if {Fα∗ Fα } is an AII for (ΦΦ∗ , T1∗ , T2 ). A ∗-approximate inverse semiintertwining (∗-AIS) is defined in a similar way: it is an AIS (= AII) {Fα } for (Φ, S, T1 ) such that {Fα∗ Fα } is an AIS for (ΦΦ∗ , T1∗ , T2 ).

118


Theorem 2.5. (i) If the pair ((Φ, S, T1 ), (Φ, S ∗ , T2 )) has a ∗-AIS, then (im T1 ) ∩ T2−1 (ΦΦ∗ (Y∗ )) ⊂ Φ(H). (ii) If ((Φ, S, T1 ), (Φ, S ∗ , T2 )) has a ∗-AII, then ||Φ−1 (T1 y)||2 = (ΦΦ∗ )−1 (T2 T1 y), y for any y ∈ (T2 T1 )−1 (ΦΦ∗ (Y ∗ )). Corollary 2.6. If ((Φ, S, T1 ), (Φ, S ∗ , T2 )) has a ∗-AIS then im T1 ∩ ker T2 = {0}. We will finish by a result which has some similarity to Theorem 2.3 but is not related to AII’s. Theorem 2.7. Let Φ : H → Y intertwine commuting normal operators S1 , S2 with operators T1 and T2 respectively. Suppose that ker(S1 ) ∩ Φ−1 (T2 Y) = {0}. Let S2 h ≤ S1 h for each h ∈ H. Then the inequality (2.1) holds for each y ∈ Y such that T1 y ∈ Φ(H) and T2 y ∈ Φ(H). Corollary 2.8. Let Φ : H → Y intertwine a normal operator S with T1 and its adjoint S ∗ with T2 . Suppose that ker(S) ∩ Φ−1 (T2 Y) = {0}. Then the inequality (2.1) holds for each y ∈ Y such that T1 y ∈ Φ(H) and T2 y ∈ Φ(H).

3. General elementary operators: the range Here by Δ we denote an elementary operator Δ(X) =

n i=1

Ai XBi

and set

n

i=1

LAi RBi on B(H):

# Δ(X) =

n

A∗i XBi∗ .

i=1

# p the restriction of Δ and respectively Δ # to the ideal Sp , We denote by Δp and Δ 1 ≤ p ≤ ∞ (by S∞ as usually we denote the ideal K(H) of all compact operators). 3.1. The intersection with the kernel of adjoint For each operator acting on a Hilbert space, the closure of its range has zero intersection with the kernel of its adjoint. We will discuss related conditions for elementary operators, that is ˜ = {0} Δ(B(H)) ∩ ker Δ (3.1) or a stronger one ˜ and some C > 0, Δ(X) + Y ≥ CY for all X ∈ B(H), Y ∈ ker Δ

(3.2)

˜ is non-zero), or weaker (which means that the angle between Δ(B(H)) and ker Δ ones ˜ = {0} Δ(B(H)) ∩ ker Δ (3.3)


119

˜ p = {0}. Δ(Sp ) ∩ ker Δ

(3.4)

and The last conditions can be rewritten as follows: ˜ = ker Δ ker ΔΔ

(3.5)

˜ p Δp = ker Δp ker Δ

(3.6)

and respectively. Note that the validity of (3.5) for a class of operators can have strong consequences. For example if it is true for an operator Δ which commutes with ˜ then ker Δ ˜ ⊂ ker Δ. On this way one can obtain various extensions of the Δ Fuglede–Putnam theorem (see Section 4). Problem 1. Is (3.5) true for inner derivations, that is for operators Δ = LA − RA ? The positive answer was known for the case that A is a weighted shift [11] or a subnormal operator with a cyclic vector [11]. Moreover it was shown in [11] that in the latter case the condition (3.1) holds. If A is normal then even (3.2) holds with C = 1 [2]. But in general (3.1) is not true for inner derivations: Anderson [3] has shown that the closure of the image of LA − RA can contain 1H . Note that the intersection of the left part of (3.3) with S1 is trivial; moreover the following much stronger condition holds: w∗

Δ(B(H)) w

˜ 1 = {0}. ∩ ker Δ

(3.7)

∗

Here Δ(B(H)) denotes the closure of Δ(B(H)) in the weak-* topology. To verify ∗ ˜ ˜ 1 then tr(X ∗ Δ(Y )) = tr(Δ(X) Y ) = 0 for each Y ∈ (3.7) note that if X ∈ ker Δ w∗

B(H). Hence tr(X ∗ Z) = 0 for each Z ∈ Δ(B(H)) w∗ ˜ 1. for X ∈ Δ(B(H)) ∩ ker Δ

. It follows that tr(X ∗ X) = 0

It would be important to prove the triviality of the intersection of the left part of (3.3) with S2 : Problem 2. For which elementary operators Δ does the equality #2 = 0 Δ(B(H)) ∩ ker Δ

(3.8)

hold? We conjecture that for all. But could it be proved at least for inner derivations? It will be shown in Section 4 that (3.8) is true (in a stronger version) for all normal elementary operators. Some conditions which are sufficient for (3.5) can be written in terms close in spirit to Voiculescu’s notion of quasidiagonality with respect to a symmetrically normed ideal [26]. We restrict ourselves to Schatten ideals Sp for simplicity.

120


Let us say that a family A = {Ak : 1 ≤ k ≤ m} of operators is p-semidiagonal if there exists a sequence of projections Pn of finite rank such that Pn → 1 in the strong operator topology, and sup ||[Ak , Pn ]||p < ∞, for each k ≤ m. n

It is clear that if p1 < p2 then each p1 -semidiagonal family is p2 -semidiagonal. In particular, 1-semidiagonality is the strongest of these conditions. Clearly, any family is ∞-semidiagonal, where · ∞ is the operator norm. Let us list some examples of p-semidiagonal families (see [22] for transparent proofs or references). 1) Any family of operators with matrices (with respect to some basis) supported by a finite number of diagonals (i.e., aij = 0 if |i − j| > m for some m ∈ N) is 1-semidiagonal. One of the simplest classes of such examples consists of families of weighted shifts. This class of examples can be considerably extended as follows. If (aij ) is a matrix of anoperator A in a basis {en}∞ n=1 , let us set |A|n = ∞ sup|i−j|=n |aij | and |A|diag = n=1 n|A|n . We say that A is diagonally bounded (with respect to the basis {en }∞ n=1 ) if |A|diag < ∞. Then: 2) Any family of diagonally bounded (with respect to the same basis) operators is 1-semidiagonal. As a consequence we obtain the following result of Voiculescu [26]: 3) Any family which belongs to the algebra of operators on L2 (T) generated by shifts u(t) → u(t − θ) and multiplication operators by twice differentiable functions f ∈ C 2 (T) (we will call the elements of these algebras “generalized Bishop’s operatos”), is 1-semidiagonal. To deduce this from 2), it suffices to calculate the matrices of shift and multiplication operators in the standard basis ek = exp(ikt), k ∈ Z. 4) It is important that all normal operators of finite multicyclicity are 2semidiagonal. More generally, if f1 , . . . , fn are Lipschitz functions on the spectrum of a normal operator A, then the family (f1 (A), . . . , fn (A)) is p-semidiagonal where p is the Hausdorff dimension of σ(A). Slightly more general, if the essential dimension of a family A = (A1 , . . . , An ) of commuting normal operators does not exceed p ≤ 2 then A is p-semidiagonal. 5) Any multicyclic almost normal operator A is 2-semidiagonal ([25]). Recall that an operator A is almost normal if its self-commutator [A∗ , A] is nuclear. Theorem 3.1. If the left coefficient family A of an elementary operator Δ is 1semidiagonal then the equality (3.3) holds. If A is p/(p − 2)-semidiagonal then (3.4) holds. Of course one could impose the same restriction (of 1-semidiagonality) on the right coefficient family B of Δ.


121

Problem 3. Find a condition which involves both coefficient families so that (3.3) or (3.4) is valid. In particular, does (3.3) hold if both A and B are 2-semidiagonal? To outline the proof of Theorem 3.1, note that the operators Δ and Δp are intertwined by the injection Φp : Sp → B(H). Similarly the injection Φp1 ,p2 : Sp1 → Sp2 , intertwines Δp1 and Δp2 for p1 ≤ p2 . If A is p-semidiagonal then one can use the projections Pn from the definition of semidiagonality, to construct an AII for these intertwinings by setting Fn (X) = Pn X. We describe firstly the results that relate properties of Δ and Δ2 : this is important because the latter acts on the # 2. Hilbert space H = S2 and its adjoint is Δ Theorem 3.2. (i) If the left coefficient family A of Δ is 2-semidiagonal then there exists an AII for (Φ2 , Δ2 , Δ). (ii) If A is 1-semidiagonal then # 2 , Δ); # (a) there exists a *-AIS for (Φ2 , Δ2 , Δ) and (Φ2 , Δ (b) there exists an AII for (Φ2 , Δ2 , Δ∞ ); # 2, Δ # ∞ ). (c) there exists a *-AII for (Φ2 , Δ2 , Δ∞ ) and (Φ2 , Δ (iii) If A is p/(p − 2)-semidiagonal, p > 2, then there exists a *-AII for # 2, Δ # p) (Φ2,p , Δ2 , Δp ) and (Φ2,p , Δ Now to prove Theorem 3.1 it suffices to apply Theorem 3.2 (parts (ii-a) and (iii)) and Corollary 2.6. 3.2. Hyponormality Let us say that an elementary operator Δ is formally positive if tr(Δ(X)X ∗ ) ≥ 0 # − ΔΔ # is formally for each X ∈ S2 . Furthermore Δ is formally hyponormal if ΔΔ positive. Theorem 3.3. Let Δ be formally hyponormal and let its left coefficient family A be 2-semidiagonal. # (i) If Δ(X) ∈ S2 , for some X ∈ B(H), then Δ(X) ∈ S2 and # Δ(X)2 ≥ Δ(X) 2. As a consequence we get # (ii) ker Δ ⊂ ker Δ. Indeed, by Theorem 3.2 (i), the assumption of 2-semidiagonality implies the existence of AII for (Φ2 , Δ2 , Δ). Since formal hyponormality of Δ means that # Δ2 is a hyponormal operator, the inequality Δ(X) 2 ≤ Δ(X)2 holds for each operator X ∈ S2 . It remains to apply Theorem 2.3 to the intertwinings (Φ2 , Δ2 , Δ) # 2 , Δ). # and (Φ2 , Δ Considering elementary operators of the form Δ = LA + RB it is easy to see # # = L[A∗ ,A] +R[B,B ∗ ] . Therefore such operator is formally hyponormal that ΔΔ−Δ Δ ∗ if A and B are hyponormal. One can also show that the converse is also true. Note that if a hyponormal operator A is multicyclic then its selfcommutator [A∗ , A] is

122


nuclear ([4]) so A is almost normal. Taking into account that multicyclic normal operators are 2-semidiagonal we apply Theorem 3.3 and obtain Corollary 3.4. Let operators A and B ∗ be hyponormal and multicyclic. Then AX − XB2 ≥ A∗ X − XB ∗ 2

(3.9)

for each operator X such that AX − XB ∈ S2 . This result from [22] was proved earlier under more restrictive conditions on A, B and X (see [7] and numerous references therein). Theorem 3.3 is related to the following result which is not restricted by hyponormality assumptions. Theorem 3.5. If A is 1-semidiagonal then # Δ(X)22 = tr(X ∗ ΔΔ(X))

(3.10)

# for each compact operator X such that ΔΔ(X) ∈ S1 . This statement can be proved in the same way as the previous results by using Theorem 2.5 (ii) and Theorem 3.2 (ii-c). To see its relation to Theorem 3.3, note that if Δ is formally hyponormal # # then clearly tr(X ∗ ΔΔ(X)) ≥ tr(X ∗ ΔΔ(X)). 3.3. Ranges of derivations and traces of commutators Now we will study which trace class operators can belong to the range of an elementary operator Δ. For this we need information about AII’s for the intertwinings (Φ1 , Δ1 , Δ) and (Φ1,p , Δ1 , Δp ). p It will be convenient, for each p ∈ (1; ∞), to denote by p the number p−1 , and set p = 1 if p = ∞, p = ∞ if p = 1. Theorem 3.6. (i) If the left coefficient family A of Δ is 1-semidiagonal then there exists an AIS for (Φ1 , Δ1 , Δ). (ii) If A is p -semidiagonal then there exists an AII for (Φ1,p , Δ1 , Δp ). Applying Theorem 2.1 we deduce from part (ii) of Theorem 3.6 Corollary 3.7. If A is p -semidiagonal then S1 ∩ Δ(Sp ) is contained in the · 1 closure of Δ(S1 ). In particular, we have the following consequence. n Corollary 3.8. Let the coefficients of Δ satisfy the condition i=1 Bi Ai = 0. If A is p -semidiagonal then tr(Δ(X)) = 0 for each operator X ∈ Sp for which Δ(X) ∈ S1 . n Indeed if Y ∈ S1 then tr(Δ(Y )) = tr( i=1 Bi Ai Y ) = 0, so the result follows from Corollary 3.7 and the continuity of trace on S1 .


123

The result can be applied to the problem “when the trace of a commutator is equal to 0?”. It is well known that a commutator [A, X] has zero trace if X is nuclear or if both operators A and X are Hilbert–Schmidt. Weiss [27] proved that the same is true if X is Hilbert–Schmidt and A is normal. The following proposition, which is an easy consequence of Corollary 3.8, widely extends this result. Corollary 3.9. Let p > 1. Suppose that {Ak }nk=1 is p -semidiagonal, Xk ∈ Sp and n k=1 [Ak , Xk ] ∈ S1 . Then n tr [Ak , Xk ] = 0. k=1

For p = ∞, this gives, for instance, that the commutator of a Hermitian operator (or a weighted shift, or a Bishop’s operator) with a compact operator has zero trace if nuclear. Taking p = 2, we obtain the same for the commutator of a normal (or almost normal) multicyclic operator with a Hilbert–Schmidt operator. The restriction on multicyclicity can be easily removed. Choosing in Proposition 3.9 for Ak the multiplication operators Mfk on L2 ([0, 1]) and for Xk the integral operators with kernels Fk ∈ L2 ([0, 1]2 ), one obtains the following result: Corollary 3.10. If fk ∈ Lip1/2([0, 1]), 1 ≤ k ≤ n, then there are no functions Fk ∈ L2 ([0, 1]2 ) satisfying the condition n (fk (x) − fk (y))Fk (x, y) = 1. (3.11) k=1

It was asked by Weiss if this is true for fk ∈ C[0, 1]; the answer is negative (see [22, Proposition 8.7] which shows that 1/2 in Corollary 3.10 cannot be even changed by 1/3). The constant p in Proposition 3.9 is sharp for p = 2, i.e., the condition Xk ∈ S2 cannot be weakened to Xk ∈ Sq for any q > 2 (see [22, Example 8.5]). Problem 4. Is the constant p sharp for all p ≥ 1?

4. Normal elementary operators In this section the coefficient families A, B of an elementary operator Δ are assumed to be commutative and to consist of normal operators. Such elementary operators are called normal. 4.1. Spectral subspaces It is well known (see for example a much more general result of [6]) that $ n σ(Δ) = λi μi : λ ∈ σ(A), μ ∈ σ(B) .

(4.1)

k=1

We will study here some other spectral characteristics of elementary operators.

124


Recall that an operator T on a Banach space X is decomposable if to each compact set α ⊂ σ(T ) there corresponds a closed subspace ET (α) invariant for all operators commuting with T and satisfying the following conditions: (a) σ(T |ET (α)) ⊂ α; (b) If U, V are open subsets of C and U ∪ V ⊃ σ(T ) then there are compact sets α ⊂ U , β ⊂ V with ET (α) + ET (β) = X . The subspaces ET (α) are called spectral subspaces of T ; the map α → ET (α) is the spectral capacity of T . To describe spectral subspaces EΔ (α) of a normal elementary operator Δ we need the following notion. Let EA (·) and EB (·) be the spectral measures of the coefficient families A and B of Δ on σ(A) and σ(B) respectively. An operator X ∈ B(H) is said to be supported by a set M ⊂ σ(A) × σ(B) if EA (U )XEB (V ) = 0 for every Borel rectangle U × V ⊂ σ(A) × σ(B) non-intersecting M . We will write in this case supp X ⊂ M . It is not difficult to check that for each subset M ⊂ σ(A)×σ(B), the subspace HM of all operators supported by M is invariant n for Δ. Moreover it can be deduced in the same way as (4.1) that σ(Δ|HM ) ⊂ { k=1 λi μi : (λ, μ) ∈ M }. For every compact α ⊂ σ(Δ), we set $ n MA,B (α) = (λ, μ) ∈ σ(A) × σ(B) : λk μk ∈ α . k=1

Set now EΔ (α) = {X ∈ B(H) : supp X ⊂ MA,B (α)}. In other words EΔ (α) = HMA,B (α) . Theorem 4.1. [18] A normal elementary operator Δ is decomposable; the map α → EΔ (α) is its spectral capacity. In particular the space EΔ ({0}) of all operators supported by MA,B (0) coincides with {X : Δn (X)1/n → 0}; in other words it is the root space of Δ. It is important that the set MA,B ({0}) has a comparatively simple structure: it is the union of a null set and a countable family of rectangles. More precisely, let μA , μB be scalar spectral measures of A and B, and let m = μA × μB be the product measure on σ(A) × σ(B). Lemma 4.2. There are measurable sets Ai ⊂ σ(A), Bi ⊂ σ(B), 1 ≤ i < ∞, and an m-null subset C of σ(A) × σ(B) such that MA,B (0) = (∪∞ i=1 Ai × Bi ) ∪ C. It follows from Lemma 4.2 that the space of all Hilbert–Schmidt operators in EΔ ({0}) is generated by the union of the spaces of all Hilbert–Schmidt operators supported by rectangles Ai × Bi . Using this one obtains easily the following result: Corollary 4.3. The space S1 ∩ EΔ ({0}) is dense in S2 ∩ EΔ ({0}) with respect to the norm of the ideal S2 .


125

Since for a normal operator on a Hilbert space the root space coincides with the kernel, the result can be reformulated as follows: ker Δ1 is a dense subspace of ker Δ2 . Now we can show that for normal elementary operators the equality (3.8) holds in a much stronger version. w∗ Let M denote the weak∗ -closure of a set M ⊂ B(H). Theorem 4.4. For every normal elementary operator Δ, Δ(B(H))

w∗

# 2 = 0. ∩ ker Δ

(4.2)

Indeed it is trivial to check that tr(Δ(X)Y ) = 0 for each X ∈ B(H) and ∗ # 1 . Hence by continuity tr(ZY ) = 0 for Z ∈ Δ(B(H))w . Y ∈ ker Δ If also Z ∈ S2 then, again by continuity, tr(ZY ) = 0 for all Y from the closure # 2 , which in # 1 in S2 . But by Corollary 4.3, this closure coincides with ker Δ of ker Δ # its turn equals ker Δ2 because Δ2 is a normal operator and Δ2 is its adjoint. So if w∗ # 2 then Z = 0. Z ∈ Δ(B(H)) ∩ ker Δ Note that we proved a more general statement: w∗

Δ(B(H))

# 2. ∩ S2 is orthogonal to ker Δ

(4.3)

From this one can easily deduce the following result of Turnˇsek [24]: Proposition 4.5. For every normal elementary operator Δ the intersection of its range with S2 is contained in the · 2 -closure of Δ(S2 ). # 2. Indeed the ·2 -closure of Δ(S2 ) is just the orthogonal complement of ker Δ ˜ p instead of Δ ˜ 2? Problem 5. For which p > 2 does the equality (4.2) hold with Δ w∗

# = 0 does not hold in general even Remark 4.6. We note that Δ(B(H)) ∩ ker Δ for normal inner derivations. One can easily construct Δ = LA − RA such that w∗

Δ(B(H))

= B(H).

4.2. Ascent The relation between the kernel of an operator T and its root space is a subject of the so-called theory of thin spectral structure. Since the root space contains the kernels of all operators T n , a natural intermediate question is the interrelations of these spaces. If the chain ker T ⊂ ker T 2 ⊂ · · · stabilizes on number m: ker T m = ker T m+1 = · · · then we say that the ascent of T equals m. We will see that the ascent m of a normal elementary operator can be estimated via the essential dimension of its left coefficient family, and that EΔ (0) = ker Δm . Theorem 4.7. If ess-dimA ≤ r then EΔ (0) = ker Δ(r/2] where (r/2] = [(r + 1)/2], the minimal integer ≥ r/2 For operators of the form X → AX + XB this result was obtained by Anderson and Foias [2]; clearly in this case it states that EΔ (0) = ker Δ.

126


The first step of the proof is to show that Δ|EΔ (0) ≤ 2|σ(B)| diam σ(A), where by |σ(B)| we mean sup{|λ| : λ ∈ σ(B)}. Hence Δk |EΔ (0) ≤ (2|σ(B)| diam σ(A))k for each k. If γ is a compact subset of σ(A) then changing Δ by ΔREA (γ) we obtain the inequality Δk REA (γ) |EΔ (0) ≤ (2|σ(B)| diam γ)k . Hence taking X ∈ EΔ (0) and setting T = Δk (X) we obtain: T EA (γ) ≤ C(diam γ)k

(4.4)

for all γ ⊂ σ(A). For each finite covering of σ(A) by disjoint Borel sets γ1 , . . . , γN , one can consider an orthogonal system {e1 , . . . , eN } of unit vectors ej supported in EA (γj ); N the condition (4.4) gives that T PN 22 ≤ C j=1 (diam γj )2k where PN is the projection onto the subspace generated by {e1 , . . . , eN }. If A is cyclic (this can be assumed without reducing generality) then such orthogonal systems can approximate a basis. It follows that if the Hausdorff dimension of σ(A) does not exceed 2k then each operator T ∈ B(H) satisfying the condition (4.4) belongs to S2 . We get that Δk (X) ∈ S2 for all X ∈ EΔ (0). Since EΔ (0) is invariant for Δ, k # 2 . By Theorem 4.4, Δk (X) = 0. Δ (X) ∈ S2 ∩ EΔ (0) = ker Δ2 = ker Δ 4.3. Fuglede type theorems ˜ if Δ is an operator The famous Fuglede–Putnam theorem says that ker Δ = ker Δ of the form X → N1 X − XN2 , where N1 , N2 are normal operators. A natural question arising in this context is whether for every normal elementary operator Δ the equations Δ(X) = 0 (4.5) and # Δ(X) =0 (4.6) are equivalent. This question was answered negatively in [19]. The construction of the counterexample is based on a modification of the famous L. Schwartz example of a set of spectral non-synthesis for the Fourier algebra of the group R3 . It is clear that EΔ (0) = HMA,B = EΔ # (0). Applying Theorem 4.7 we obtain the following version of the Fuglede-Putnam theorem: # (r/2] . Theorem 4.8. If ess-dimA ≤ r then ker Δ(r/2] = ker Δ The following consequence of Theorem 4.8 can be also immediately deduced from Theorem 3.3 (ii). Theorem 4.9. If ess-dimA ≤ 2 then equations (4.5) and (4.6) are equivalent. It is reasonable to regard the work around Fuglede Theorem in a wider way: as the study of conditions for the coincidence or inclusion of kernels of elementary operators. In other words: which linear operator equations are equivalent, how


127

the solution spaces of different equations are related? The following theorem gives information for the case of 2-dimensional coefficient families. Lipschitz functions on σ(A), Theorem 4.10. Let ess-dimA ≤ 2. Let {fi}m i=1 be m {gi }m be Borel functions on σ(B). Assume that i=1 i=1 fi (λ)gi (μ) = 0 for each n (λ, μ) ∈ σ(A) × σ(B) satisfying the condition k=1 λk μk = 0. Then each solution X of the linear operator equation n

Ak XBk = 0

(4.7)

fi (A)Xgi (B) = 0.

(4.8)

k=1

satisfies the equation m i=1

Clearly Theorem 4.9 is a special case of Theorem 4.10 – it corresponds to the functions fi (λ) = gi (λ) = λi . To deduce Theorem 4.10 from Theorem 4.7, denote by f (A) and g(B) respectively mthe families {fi (A) : 1 ≤ i ≤ m} and {gi (B) : 1 ≤ i ≤ m}. Let Δ = i=1 Lfi (A) Rgi (B) . In our assumptions MA,B (0) ⊂ Mf (A),g(B) (0) whence EΔ (0) ⊂ EΔ (0). Furthermore ess-dim f (A) ≤ 2, so EΔ (0) = ker Δ by Theorem 4.7. Clearly the “Fuglede theorem”, for arbitrary normal Δ, holds in S2 (that is the equations (4.5) and (4.6) are equivalent in S2 ) and hence in Sp , p < 2. Problem 6. Let Δ be a normal elementary operator. Are equations (4.5) and (4.6) equivalent in Sp , 2 < p ≤ ∞? One can ask about the validity of similar results in C ∗ -algebras different from B(H). Namely, let A be a C ∗ -algebra. Consider elementary operators Δ on A, i.e., n Δ(c) = k=1 ak cbk , where ak ,bk ∈ A. We say that Δ is normal if {ak }nk=1 and {bk }nk=1 are families of commuting normal elements in A. ˜ = ker Δ Let us call a C*-algebra A 1-Fuglede (or (F1), for short) if ker Δ 2 for any normal elementary operator Δ. If ker Δ = ker Δ for any normal Δ then ˜ = ker Δ for any Δ then A is said to be A is said to be 2-Fuglede (F2). If ker ΔΔ 3-Fuglede (F3). Clearly (F3) =⇒ (F2) =⇒ (F1). It follows from the above discussion that the algebra B(H) is not (F1) (for any infinite-dimensional H). It is not difficult to see that (i) the algebra, S∞ , of compact operators is (F2); (ii) as a consequence each CCR-algebra is (F2); (iii) each C*-algebra with a faithful family of traces (for example a simple unital hyperfinite C*-algebra) is 3-Fuglede. Problem 7. Which C ∗ -algebras A are F1, F2 and F3? Is S∞ 3-Fuglede? Is any GCR-algebra (F2)? What about the Calkin algebra B(H)/S∞ ?

128


We will briefly list other extensions of the Fuglede Theorem. E.A. Gorin [8] discovered that if in the classical Rosenblum’s proof of the Fuglede Theorem one uses instead of the Liouville theorem the Phragmen–Lindelöf Theorem, then the result extends to a more general class of operators than the normal ones. Theorem 4.11. Let an operator A, acting on a Banach space X, be decomposed into a sum A = T + iS, where [S, T ] = 0, S has real spectrum and T satisfies the condition exp(itT ) = o(|t|) for t → ∞. (4.9) Then ker A = ker T ∩ ker S. Applying this theorem to multiplication operators we obtain Corollary 4.12. Let an operator N on a Banach space X can be written as a sum N = U + iV where [U, V ] = 0, V has real spectrum and U satisfies condition exp(itU ) = o(|t|1/2 ).

(4.10)

Set N = U − iV . Then the equations N X − XN = 0 and N X − XN = 0 are equivalent. An operator T on a Banach space is called Hermitian if exp(itT ) = 1 for all t ∈ R. The following result belongs to K.Boyadzhiev [5]. Theorem 4.13. Let P (t1 , . . . , tn ) be a polynomial in n variables without zeros in Rn \{0}. If T1 , . . . , Tn are commuting Hermitian operators then ker P (T1 , . . . , Tn ) ⊂ ∩nk=1 ker Tk . To deduce the Fuglede Theorem from Theorem 4.13, one should take P (t1 , t2 ) = t1 + it2 , T1 = LA − RA , T2 = LB − RB where A, B are the real and imaginary parts of a normal operator N ∈ B(H). E.A. Gorin [8] obtained also the following “non-commutative” version of the Fuglede Theorem. Let F2 be the free algebra with generators a, b. Consider the formal power series f (z) = exp(za) exp(zb) with coefficients in F2 . Then there is a formal power series g(z) with coefficients in F2 such that f (z) = exp(g(z)). Let us denote by ck (a, b) the coefficients of this power series: g(z) = c0 (a, b) + c1 (a, b)z + · · · . It is easy to obtain explicit formulas for ck (a, b) for small k. For example c0 (a, b) = 1, c1 (a, b) = a + b, c2 (a, b) = [a, b]/2. Since ck (a, b) are elements of F2 (“non-commutative polynomials”), one can calculate ck (A, B) for each pair A, B of elements of any algebra. Theorem 4.14. Let T , S be operators on a Banach space X such that T satisfies (4.10), S has real spectrum, and let X ∈ B(X). If [ck (T, iS), X] = 0 for all k ∈ N then [T, X] = [S, X] = 0.


129

To see that this theorem extends Corollary 4.12 note that if [a, b] = 0 then ck (a, b) = 0 for all k ≥ 2. Hence in this case the assumption becomes just [c1 (a, ib), x] = 0, that is a + ib commutes with x. 4.4. Norm inequalities The following result is a special case of Theorem 3.3 (i). # ∈ S2 and Theorem 4.15. Suppose that ess-dimA ≤ 2. If Δ(X) ∈ S2 then Δ(X) # Δ(X)2 = Δ(X) (4.11) 2. It extends the famous result of Gary Weiss [29] which states that (4.11) holds for Δ = LN1 − RN2 where Ni are normal operators. The arguments similar to ones used in Theorem 3.3 allow to extend Theorem 4.15 in spirit of Theorem 4.10: be Lipschitz functions Theorem 4.16. Let ess-dimA ≤ 2. Let {fi}m i=1 non σ(A), m {gi }m be Borel functions on σ(B). Assume that | f (λ)g (μ)| ≤ | i i=1 i=1 i k=1 λk μk | for each (λ, μ) ∈ σ(A) × σ(B). If nk=1 Ak XBk ∈ S2 , for some X ∈ B(H), then m i=1 fi (A)Xgi (B) ∈ S2 and m n fi (A)Xgi (B) ≤ Ak XBk . i=1

2

k=1

2

Kittaneh [14] established the following special case of this result: if A is a normal operator and f is a Lipschitz function on σ(A) then f (A)X − Xf (A)2 ≤ k(f )AX − XA2

(4.12)

where k(f ) is the Lipschitz constant of f . Without restrictions on the dimension of spectra the equality (4.11) can fail # can differ. Nevertheless Weiss [28] proved the because even the kernels of Δ and Δ following remarkable result: # Theorem 4.17. Let Δ be a normal elementary operator. If both Δ(X) and Δ(X) are Hilbert–Schmidt operators then (4.11) holds. One can prove Theorem 4.17 applying Corollary 2.8. Take as usually H = S2 , # then the identity inclusion Φ2 : S2 → B(H) S = Δ2 , Y = B(H), T1 = Δ, T2 = Δ, intertwines the operators. The condition ker S ∩ Φ−1 (T2 Y) = 0, which is just # = 0, holds by Theorem 4.4. So inequality (2.1) holds which ker Δ2 ∩ Δ(B(H)) # # # ∈ S2 . Interchanging Δ means that Δ(X)2 ≤ Δ(X)2 if Δ(X) ∈ S2 and Δ(X) and Δ we obtain (4.11). Using Theorem 2.7 instead of Corollary 2.8 one can obtain the following more general result: Theorem 4.18. Let {fi}m Lipschitz functions onσ(A), {gi }m i=1 be i=1 be Borel funcm n tions on σ(B). Assume that | i=1 fi (λ)gi (μ)| ≤ | k=1 λk μk | for each (λ, μ) ∈ σ(A) × σ(B).

130

V.S. Shulman and L. Turowska If

then

n k=1

Ak XBk ∈ S2 and

m i=1

fi (A)Xgi (B) ∈ S2 , for some X ∈ B(H),

m n fi (A)Xgi (B) ≤ Ak XBk . i=1

2

k=1

2

The mentioned result of (4.11) extends to other Schatten ideals: for every p ∈ (1, ∞) there is a constant c = cp > 0 such that A∗ X − XA∗ p ≤ cAX − XAp

(4.13)

for all normal operators A and all operators X (Abdessemed and Davies [1], for p > 2, Shulman [20], for p < 2). This is not the case for p = 1 and p = ∞ (Yu.B. Farforovskaya [10]). Kissin and Shulman [12] proved that the statement remains true for the operator norm and S1 -norm, if one imposes a restriction on spectra of normal operators. Namely, for each C 2 -smooth Jordan line L there is a constant c = cL such that A∗ X − XA∗ ≤ cAX − XA (4.14) if σ(A) ⊂ L. It was also proved in [12] that a kind of smoothness of spectrum is necessary for the validity of (4.14): if for some normal operator A the inequality (4.14) holds for all X, then given a sequence λn ∈ σ(A) converging to λ ∈ σ(A), there is a limit lim(λn − λ)/|λn − λ|. Potapov and Sukochev [15] using a powerful technique of Banach space geometry and harmonic analysis proved that (4.12) extends to all Sp ideals, if A = A∗ . For all normal A this was proved in [13]. If we note that for each X, the map δX : A → AX − XA is a derivation of the algebra B(H) then the question arises if the above results can be extended to derivations of more general class (defined on subalgebras of B(H)). The following theorem was proved in [20]. Theorem 4.19. Let δ be a derivation from a *-subalgebra D(δ) ⊂ B(H) to B(H), and let A ∈ D(δ) be a normal operator without eigenvalues. Then (i) If δ(A) ∈ Sp and δ(A∗ ) ∈ K(H) then δ(A∗ ) ∈ Sp and δ(A∗ )p ≤ 2cp δ(A)p ;

(4.15)

(ii) if moreover p = 2 and δ is closed then δ(f (A))2 ≤ k(f )δ(A)2

(4.16)

for each Lipschitz function f on σ(A) with Lipschitz constant k(f ). The extension of part (ii) to all p ∈ (1, ∞) was obtained in [13]. Let A ∈ B(H) be a normal operator. As it was mentioned above the map AX − XA → A∗ X − XA∗ is not bounded in general as a map on B(H). However this map and more general maps of the form AX − XA → f (A)X − Xf (A) (f is a measurable function) are norm closable. In fact, if AXn − Xn A → 0 and f (A)Xn − Xn f (A) → B as n → ∞ then [B, A] = lim [[f (A), Xn ], A] = lim [f (A), [Xn , A]] = 0. n→∞

n→∞


131

Thus B is in the commutant {A} of A. Hence B commutes with f (A) and therefore belongs to δf (A) (B(H)) ∩ ker δ f (A) which is {0} by [2]. It would be interesting to see what happen if we weaken the condition on the topology replacing it by the weak-∗ -topology on B(H). It was proved in [21] that the Fuglede map AX − XA → A∗ X − XA∗ is not w∗ -closable if σ(A) has nonempty interior and the spectral measure of A is equivalent to the Lebesgue measure on the interior of σ(A). The proof uses a characterization of so-called Toeplitz w∗ closable multipliers given in [21]. However the method does not work for general maps of the type AX − XA → f (A)X − Xf (A). Some sufficient conditions are established in [21]. Problem 8. For which continuous functions f and normal operators A ∈ B(H) is the map on B(H) given by AX − XA → f (A)X − Xf (A) closable in weak-∗ topology?

References [1] A. Abdessemed, E.B. Davies, Some commutator estimates in the Schatten classes, J. London Math. Soc. (2) 39 (1989), no. 2, 299–308. [2] J. Anderson and C. Foias, On properties which normal operators share with normal derivations and related operators, Pac. J. Math. 61 (1975), 313–325. [3] J. Anderson, Derivation ranges and the identity, Proc. Amer. Math. Soc. 38 (1973), 135–140. [4] C.A. Berger B.I. Shaw, Selfcommutators of multicyclic hyponormal operators are always trace class, Bull. Amer. Math. Soc. 79 (1973), no. 6, 1193–1199. [5] K. Boyadziev,Commuting C0 -groups and the Fuglede–Putnam theorem, Studia Math. 81 (1985), no. 3, 303–306. [6] R. Curto and L. Fialkow, The spectral picture of [LA , RB ], J. Funct. Anal. 71 (1987), 371–392. [7] M.R. Duggal, On Fuglede-Weiss theorem, Indiana Univ. Math. J. 36 (1987), 526– 534. [8] E.A. Gorin, A generalization of a theorem of Fuglede, Algebra and Analysis, 5 (1993), 83–97. [9] E.A. Gorin, M.I. Karahanyan, Asymptotic version of Fuglede–Putnam Theorem on commutators for elements of Banach algebras, Mathem. Zametki 22 (1977), no. 2, 179–188. [10] Yu.B. Farforovskaya, Example of a Lipschitz function of selfadjoint operators that gives a non-nuclear increment under a nuclear perturbation, J. Soviet Math. 4 (1975), 426–433. [11] Y. Ho, Commutants and derivation ranges, Tohoku Math. J. 27 (1975), 509–514. [12] E. Kissin and V.S. Shulman, Classes of operator-smooth functions. I. OperatorLipschitz functions, Proc. Edinburgh Math. Soc. 48 (2005), 151–173. [13] E. Kissin, D. Potapov, V. Shulman and F. Sukochev, Lipschitz functions, symmetrically normed ideals and unbounded derivations, in preparation.

132


[14] F. Kittaneh, On Lipschitz functions of normal operators, Proc. Amer. Math. Soc. 94 (1985), 416–418. [15] D. Potapov and F. Sukochev, Operator-Lipschitz functions in Schatten-von Neumann classes, Acta Math., to appear. [16] C. Putnam, On normal operators on Hilbert space, Amer. J. Math. 73 (1951), 357– 362. [17] M. Rosenblum, On a theorem of Fuglede and Putnam, J. London Math. Soc. 33 (1958), 376–377. [18] V.S. Shulman, Linear operator equations with generalized scalar coefficients, (Russian) Dokl. Akad. Nauk SSSR 225 (1975), no. 1, 56–58. [19] V.S. Shulman, Multiplication operators and spectral synthesis, Doclady AN SSSR 313 (1990), no. 5, 1047–1051. [20] V.S. Shulman, Some Remarks on the Fuglede-Weiss Theorem, Bull. London Math. Soc. 28 (1996), 385–392. [21] V.S. Shulman, I.G. Todorov and L. Turowska, Closable multipliers, to appear in Integral Equations Operator Theory, arXiv: 1001.4638. [22] V. Shulman and L. Turowska, Operator synthesis II: Individual synthesis and linear operator equations J. Reine Angew. Math. 590 (2006), 143–187. [23] V. Shulman and L. Turowska, Beurling-Pollard type theorems, J. Lond. Math. Soc. (2) 75 (2007), no. 2, 330–342. [24] A. Turnˇsek, On the range of elementary operators, Publ. Math. Debrecen 63 (2003), no. 3, 293–304. [25] D. Voiculescu, Remarks on Hilbert-Schmidt perturbation of almost-normal operators. Topics in modern operator theory (Timi¸soara/Herculane, 1980), pp. 311–318, Operator Theory: Adv. Appl., 2, Birkh¨ auser, Basel-Boston, Mass., 1981. [26] D. Voiculescu, Some extensions of quasitriangularity, Rev. Roumaine Math. Pures Appl. 18 (1973), 1303–1320. [27] Gary Weiss, The Fuglede commutativity theorem modulo the Hilbert–Schmidt class and generating functions for matrix operators. I, Trans. Amer. Math. Soc. 246 (1978), 193–209. [28] Gary Weiss, An extension of the Fuglede–Putnam theorem modulo the Hilbert– Schmidt class to operators of the form Mn XNn Trans. Amer. Math. Soc. 278 (1983), no. 1, 1–20. [29] Gary Weiss, The Fuglede commutativity theorem modulo the Hilbert–Schmidt class and generating functions for matrix operators. II, J. Operator Theory 5 (1981), no. 1, 3–16. V.S. Shulman Department of Mathematics, Vologda State Technical University, Vologda, Russia e-mail: shulman [email protected] L. Turowska Department of Mathematical Sciences, Chalmers University of Technology and the University of Gothenburg, Gothenburg SE-412 96, Sweden e-mail: [email protected]

Computation Versus Formulae for Norms of Elementary Operators Richard M. Timoney Abstract. A long-standing problem was to find a formula for the norm of an elementary operator acting on a C ∗ -algebra (or a matrix algebra). As with the best problems, the problem was quick to explain, although there was no conjectured answer. The solution which has been found involves concepts that are almost equally simple to explain, but the effective use of the formula is more subtle. It does lead to insights into the structure of the algebras. The techniques involved in justifying the formula are useful in practical and theoretical ways. Mathematics Subject Classification (2000). Primary 47B47; Secondary 46L07, 46L05. Keywords. Completely bounded norm; matrix numerical range; tracial geometric mean; primal ideal; Glimm ideal; factorial state; pure state; central Haagerup tensor product.

1. Preamble Elementary operators are linear operators on algebras which are defined in an appealingly simple algebraic way. In view of the very significant amount of research on them, and the results that have been obtained in different directions, the term ‘elementary’ may be a bit of a misnomer. Our aim here is to give a flavour of some of the work on the topic, but to concentrate on results about norms of elementary operators acting on C ∗ -algebras, the simplest examples being the algebras Mn = Mn (C) of n × n complex matrices. Recall that, by definition, an elementary operator on a unital algebra A is a map T : A → A which is expressible as a finite sum of two-sided multiplication maps x → axb. We will write E(A) for the elementary operators on A. Thus each


133

134

R.M. Timoney

T ∈ E(A) is expressible as Tx =

aj xbj

(1.1)

j=1

where ≥ 1 and aj and bj belong to A. (We should allow the trivial case T = 0 and = 0, but it is also included with = 1, a1 = b1 = 0.) Left multiplication operators La (x) = ax (with a ∈ A) and right multiplications Rb (x) = xb (with b ∈ A) are included, and perhaps this is already an indication that the whole structure of the algebra is somehow reflected by properties of the elementary operators on it. We will explain substantive results that connect the structure of an algebra with the elementary operators on it, but there are many more results that explain the interest in these operators. Some results may not even refer to the terminology, such as results that all derivations on certain algebras are inner, which have cohomological interpretations. In quantum information theory the term ‘superoperator’ is used for operators on algebras. At least for A = Mn , these are all elementary operators. For unital Banach algebras A, the terminology ‘elementary operator’ seems to have been coined by Lumer and Rosenblum [21], who required that the sets {aj : 1 ≤ j ≤ } and {bj : 1 ≤ j ≤ } separately commute. Their concerns were largely with the spectrum of T , and they were interested in generalising Sylvester’s theorem concerning solutions x of matrix equations (La − Rb )(x) = y. We refer to the article [9] for a wide-ranging survey of the consequences and applications of Sylvester’s work. The work of Fong and Sourour [14] is an early substantial contribution where the commutativity of the sets {aj } and {bj } is not required. The first workshop in the series, held in 1991 (proceedings volume [25]) reflected substantial progress during the preceding decade, and the second was held at the University of Helsinki, Finland during September, 2001. Rather than attempting to survey the different developments considered by various workers (as is done in [28]), we concentrate here on a rather specific strand aimed at finding the norm of T as in (1.1) for the case of a C ∗ -algebra A. This problem was stated explicitly in [25] and there is a comprehensive survey by Mathieu [26] of progress in the previous decade. Perhaps the problem can be considered to have been solved recently. Except in the last two sections, A will always be a unital C ∗ -algebra. When we refer to an ideal, we always mean a closed two-sided ideal.

2. Observations It is clear that if I ⊂ A is an ideal and T ∈ E(A), then T (I) ⊂ I. Moreover, with T as in (1.1), T induces T I ∈ E(A/I) via T I (x + I) = j=1 aj xbj + I = T x + I. It seems that the property of preserving ideals of A, which is almost tautological, underlies many of the deeper results linking properties of elementary operators to the ideal structure of A. Magajna [22, Corollary 2.3] showed that

Computation Versus Formulae

135

in the (relatively weak) sense of point norm convergence, all bounded linear operators on A that preserve ideals are limits of elementary operators. (There are characterisations of C ∗ -algebras A where stronger approximation results hold in [24, 23].) We use B(H) for the bounded linear operators on a Hilbert space H, and write ˆ A for the unitary equivalence classes [π] of irreducible representations π : A → ˆ we can define T ker π ∈ E(A/ ker π) B(Hπ ). For T ∈ E(A) as in (1.1) and π ∈ A, using the ideal I = ker π. Essentially equivalently we can use the natural isomorphism A/ ker π ∼ = π(A) to treat this as an elementary operator on π(A). Instead we will use the operator T π ∈ E(B(Hπ )) which is given by π

T (y) =

π(aj )yπ(bj )

(y ∈ B(Hπ )),

j=1

Since T π is continuous in the weak* (or ultraweak) topology of B(Hπ ), T π restricts to the canonical element of E(π(A)) associated with T , and π(A) is weak*-dense in B(Hπ ), we can see that T π is independent of the representation (1.1) of T ∈ E(A). From the Kaplansky density theorem T π is the same as the norm of its restriction to π(A) and then faithfulness of the reduced atomic decomposition implies that T = sup T π .

(2.1)

ˆ π∈A

See [1, Theorem 5.3.12] for a more general result than (2.1). We can see from (2.1) that we could calculate T for T ∈ E(A) if we knew how to deal with the case A = B(H). We will first consider a solution by Stampfli [30] for the special case of inner derivations.

3. Inner derivations on A = B(H) For c ∈ A, the inner derivation δc ∈ E(A) is given by δc (x) = xc − cx. It is probably not necessary to mention that derivations in general and inner derivations in particular play an important role in many ways. Their connection to Lie theory and the Leibniz rule they satisfy partly explains the interest in them. An obvious estimate is that δc ≤ 2c. Since δc−λ = δc we have δc ≤ 2 inf λ∈C c − λ (where c − λ means c − λ1A with 1A ∈ A the identity). Theorem 3.1 (Stampfli [30]). For A = B(H), δc = 2 inf λ∈C c − λ. We contend that the importance of this result stems not just from the elegance of the formula, but also from the method of proof. The proof used a modification of the numerical range of c − λ to characterise the λ ∈ C where the infimum is attained. This characterisation implies an algorithm for checking any guess one might be able to make about the infimum. Recall that the usual (spatial) numerical range of c ∈ B(H) is W (c) = {cξ, ξ : ξ ∈ H, ξ = 1},

136

R.M. Timoney

where ·, · denotes the inner product on H (which we take to be linear in the first variable). Standard facts about W (c) are that it is a convex subset of C (the Toeplitz–Hausdorff theorem), it need not be closed (unless dim H < ∞), the closure W (c) contains the spectrum of c, and W (c) coincides with the convex hull of the spectrum of c if c is normal (c∗ c = cc∗ ). (See [18] for instance.) Stampfli [30] used a subset W0 (c) ⊂ W (c) which can be conveniently explained using the joint numerical range of (c∗ c, c). For a tuple (c1 , c2 , . . . , cn ) of operators on H, their joint numerical range is W (c1 , c2 , . . . , cn ) = {(cj ξ, ξ)nj=1 : ξ ∈ H, ξ = 1} ⊂ Cn . Using the closure of W (c∗ c, c), we can describe W0 (c) = {z ∈ C : (c2 , z) ∈ W (c∗ c, c)}. Stampfli [30] showed that (for c ∈ B(H)) δc = 2c − λ ⇐⇒ 0 ∈ W0 (c − λ).

(3.1)

We cannot express W0 (c − λ) in terms of W0 (c) and λ, but a very simple calculation reveals that z λ), z − λ) : (x, z) ∈ W (c∗ c, c)}. W ((c − λ)∗ (c − λ), c − λ) = {(x + |λ|2 − 2(¯ Thus W0 (c − λ) must be computable from the joint numerical range W (c∗ c, c). Also the norm c is given by c2 = sup{x : (x, z) ∈ W (c∗ c, c) for some z ∈ C}, and W (c∗ c, c) ⊂ R × C contains all the information needed to identify δc . Our aim here is to explain a generalisation of this strategy that applies to T ∈ E(B(H)). We should point out that we are omitting discussion of many developments arising from Stampfli’s paper relating to δc for c ∈ A, A more general than B(H). However, those aspects are rather well summarised in [1, §4.1, 4.6].

4. Haagerup estimates Consider now T ∈ E(B(H)) given as in (1.1) with aj , bj ∈ B(H) (1 ≤ j ≤ ). Following an idea of Haagerup [17], we express T x as a 1 × 1 matrix with entry in B(H) and as a (block) matrix product ⎡ ⎤⎡ ⎤ b1 x 0 ··· 0 ⎥ ⎢ b2 ⎥ 0 x 0 ⎢ ⎢ ⎥⎢ ⎥ T x = a1 a2 . . . a ⎢ . ⎥ ⎢ .. ⎥ = ax() b, .. ⎣ .. ⎦⎣ . ⎦ . 0

0

···

x

b

where a denotes the row, x() denotes the block diagonal with copies of x on the diagonal, and b denotes the column. Treating this as a factorisation of T x through


137

H → H → H → H (with H the Hilbert space direct sum of copies of H), we get + , , ∗ ∗ T x ≤ abx = aj aj bj bj x. j=1

j=1

An important part of Haagerup’s contribution in [17] was that the resulting estimate for T is also an estimate for potentially bigger norms associated with T . We can identify k × k block matrices X = (xr,s )kr,s=1 with entries xr,s ∈ B(H), that is the elements of Mk (B(H)), with operators on H k by treating vectors in H k as columns (k × 1 matrices with entries in H). In summary, we have Mk (B(H)) ∼ = B(H k ). We associate with any linear operator T : B(H) → B(H) the operators T (k) : Mk (B(H)) → Mk (B(H)) given by T (k) (X) = T (k) ((xr,s )kr,s=1 ) = (T xr,s )kr,s=1 . In tensor notation, we could regard Mk (B(H)) as B(H) ⊗ Mk and then T (k) becomes T ⊗ id (where id means the identity on Mk ). We define T k = T (k) . For T bounded, we do always have T k < ∞ for each k. However, a considerable ingredient of modern operator space theory is based on the fact operators where supk≥1 T k < ∞, known as completely bounded operators, have a special significance. The completely bounded norm (or CB-norm) of such T is def

T cb = sup T k . k≥1

For T ∈ E(B(H)) as in (1.1), it is easy to see that T (k) ∈ E(Mk (B(H))) (for each k ≥ 1) because we can write T (k) (X) =

(k)

(k)

aj Xbj

(4.1)

j=1 (k)

(k)

(using the same notation for aj and bj as we used above for the block diagonal x() ). If we now use the block matrix trick again, with T replaced now by T (k) , we get T k ≤ ab, an estimate independent of k and hence a bound for T cb. There is a further avenue to improve the estimate. Though we begin with T expressed in the form (1.1), there are many such expressions giving T . We may consider the tensor aj ⊗ bj ∈ B(H) ⊗ B(H) u= j=1

and then there is a well-defined linear map θ : B(H) ⊗ B(H) → E(B(H)) given by θ(u) = T , or θ(u)(x) = aj xbj (x ∈ B(H)). j=1

138

R.M. Timoney

The tensor norm defined by Haagerup [17] is now known as the Haagerup tensor norm and is defined (on B(H) ⊗ B(H)) by ⎧+ ⎫ ⎬ $ ⎨, , uh = inf ab : u = aj ⊗ bj = inf - aj a∗j b∗j bj ⎭ . ⎩ j=1

j=1

j=1

The insight of Haagerup was that, not only do we get a bound on T cb, we actually get equality. Theorem 4.1 (Haagerup [17]). For u = j=1 aj ⊗ bj ∈ B(H) ⊗ B(H) and T = θ(u) ∈ E(B(H)), we have T cb = uh . Haagerup’s argument can be found in [1, §5.4]. It would be hard to overstate the influence of this contribution by Haagerup, but we will limit ourselves to some of its consequences for elementary operators. One of the early discoveries was that the infimum in the definition of uh is attained, and moreover that if u = j=1 a ˜j ⊗ ˜bj then there is another expression u = j=1 aj ⊗ bj where 1 (a2 + b2 ) (4.2) 2 ak : 1 ≤ k ≤ }), bj ∈ span({˜bk : 1 ≤ k ≤ }). This fact is and aj ∈ span({˜ usually summarised as ‘injectivity’ of the Haagerup tensor norm (in the context of operator spaces). There are by now at least 4 authoritative recent monographs on operator space theory, one of which is Effros and Ruan [12]. We refer to those for details missed out above. uh = ab =

5. Algorithms While it is very useful to have Haagerup’s Theorem and the fact that equality is attainable via expressions of u ∈ B(H) ⊗ B(H) within a specific finite-dimensional space, this does not necessarily provide a direct way to find uh in any given example. It seems necessary to search for lower bounds for T cb ≥ T k with arbitrarily large k and upper bounds of the form ab. In [31] a number of results were established that help in this respect, via the use of (joint) numerical range considerations. These results were inspired by the considerations of Stampfli [30] outlined in §3. The joint numerical ranges are those of the 2 -tuples {ai a∗j : 1 ≤ i, j ≤ } and {b∗i bj : 1 ≤ i, j ≤ } but it is convenient to consider them as subsets of the space of × matrices. We define the matrix numerical range Wm (b) associated with the column b by Wm (b) = {(b∗j bi ξ, ξ)i,j=1 : ξ ∈ H, ξ = 1}.

(5.1)

We will also consider the subset of the closure with maximal trace b2 , which is Wm,e (b) = {X ∈ Wm (b) : trace(X) = b2 }. Similarly for the row a we apply adjoints to get a column a∗ and consider Wm (a∗ ) and Wm,e (a∗ ) = {Y ∈ Wm (a∗ ) : trace(Y ) = a2 }.


139

Theorem 5.1 (Theorem 1). For u = i=1 ai ⊗ bi ∈ B(H) ⊗ B(H), T = θ(u), we have 1 T = (a2 + b2 ) = uh ⇐⇒ Wm,e (a∗ ) ∩ Wm,e (b) = ∅. 2 Recall that T ≤ T cb and strict inequality is to be expected in general. Thus, although we can always rewrite the tensor u so that (4.2) holds (and also we have uh = T cb), this theorem cannot always be applied directly. However, let us now recall (4.1). Write a(k) for the row associated with (4.1) and b(k) for the column. Then Wm (b(k) ) = cok (Wm (b) is easy to verify, where cok (·) means the set of all convex combinations of at most k elements of a set. Similarly Wm ((a(k) )∗ ) = cok (Wm (a∗ ). It is then a short step from Theorem 1 to get the following. Theorem 5.2 (Theorem k). For k ≥ 1 and u, T as in Theorem 1, we have 1 T k = (a2 + b2 ) = uh ⇐⇒ cok (Wm,e (a∗ )) ∩ cok (Wm,e (b)) = ∅. 2 More or less by letting k become large, we get the following. Here co(·) will denote the convex hull of a set. Theorem 5.3 (Theorem CB). Let u and T be as in Theorem 1. We have 1 (a2 + b2 ) = uh ⇐⇒ co(Wm,e (a∗ )) ∩ co(Wm,e (b)) = ∅. 2 Moreover T k = T cb for k = min(, dim H). The proof that k = suffices for T k = T cb uses a generalisation of the classical Toeplitz–Hausdorff theorem to tuples. It is not true that the joint numerical range W of more than one operator on H needs to be convex, but such numerical ranges satisfy cok (W ) = co(W ) for a smaller value of k than holds for arbitrary subsets of a vector spaces with the appropriate dimension. (2 is the real dimension here. See [31, Remark 2.5].) ˜j ⊗ ˜bj , we know we can find another expression Starting with u = j=1 a ˜ as shorthand for the row and column ˜ and b u = j=1 aj ⊗ bj as in (4.2). We use a associated with the a ˜j and ˜bj (by analogy with a and b). But then Wm (a∗ ) is expressible in terms of the matrix numerical range Wm (˜ a∗ ). The norm a is also computable from Wm (a∗ ). Similarly Wm (b) and b are expressible in terms of ˜ So, at least in principle, T cb = uh is computable in terms of the Wm (b). ˜ a∗ ) and Wm (b). matrix (or joint) numerical ranges Wm (˜

6. Formula for the norm on B(H) This last fact does not yet allow us to claim that T is computable in terms of these same joint numerical ranges. We now continue to deal here with u = j=1 aj ⊗ bi ∈ B(H) ⊗ B(H) and the associated T ∈ E(B(H)) given by (1.1).

140

R.M. Timoney

A device used in [32] was to write T in terms of norms of ‘rank one’ elementary operators, where the norm and CB norm coincide. Hence the norms of these rank one elementary operators are accessible via the numerical range techniques discussed above. The idea is that T = sup{x → (T x)η, ξ : ξ, η ∈ H, ξ = η = 1}, as one can easily verify. This can be reformulated T = sup{x → p1 (T x)p2 : p2i = pi = p∗i ∈ B(H) of rank = 1(i = 1, 2)}. Using Theorem 1, it is possible to compute the norm of x → p1 (T x)p2 explicitly. The result is expressible in terms of a quantity called in [32] the tracial geometric mean of two positive semidefinite × matrices X and Y . It is given by tgm(X, Y ) = trace((X 1/2 Y X 1/2 )1/2 ),

(6.1)

1 2

where the exponent means the positive semidefinite square root of the matrices. Here is the result. (We also use the notation (5.1) again). Theorem 6.1 (Formula 1 [32]). For u = i=1 ai ⊗ bi ∈ B(H) ⊗ B(H), T = θ(u), we have T = sup{tgm(X, Y ) : X ∈ Wm (a∗ ), Y ∈ Wm (b)}. Using the same techniques that led from Theorem 1 to Theorem k above, we get the following. Theorem 6.2 (Formula k [32]). For k ≥ 1 and u, T as in Formula 1, we have T k = sup{tgm(X, Y ) : X ∈ cok (Wm (a∗ )), Y ∈ cok (Wm (b))}. Theorem 6.3 (Formula CB [32]). For u, T as in Formula 1, we have T cb = uh = sup{tgm(X, Y ) : X ∈ co(Wm (a∗ )), Y ∈ co(Wm (b))}. Example. We might then be tempted to try Formula 1 on inner derivations δc , c ∈ B(H). The result is ∗ 1 −c∗ ξ, ξ c cη, η cη, η δc = sup tgm , , −cξ, ξ cc∗ ξ, ξ c∗ η, η 1 (with the supremum over unit vectors ξ, η ∈ H) but this does not seem to allow any easy way to “re-discover” Stampfli’s formula from §3 above. Remark 6.4. Although it was not noticed by the author in [32], the quantity tgm(X, Y ) has been used extensively under the name fidelity in the literature on quantum information theory (at least when X and Y have trace 1). A source is [27, §9.2.2] (and references therein). One might hope that the different perspectives of operator space theory (which is where the Haagerup tensor norm has been of fundamental importance) and quantum information theory would lead to new results in one area inspired by the other. Perhaps [33] gives some evidence for this possibility.


141

7. Formula for C ∗-algebras In view of (2.1), it is not difficult to generalise Formula 1 and Formula k to the case of T ∈ E(A), for general (unital) C ∗ -algebras A. It helps to think in the language of states. On B(H), for ξ ∈ H a unit vector, we have the associated state ωξ given by ωξ (x) = xξ, ξ (for x ∈ B(H)). Then ωξ is a pure state of B(H) (in fact a pure normal state). We write S(A) for the states on a C ∗ -algebra A (the positive linear functionals of norm 1), and P(A) for the pure states of A (the extreme points of S(A)). Given φ ∈ S(A) and an -tuple b = (b1 , b2 , . . . , b ) of elements of A (which we think of as a column matrix having entries in A), we define Q(b, φ) = (φ(b∗j bi ))i,j=1 . Using this notation, when A = B(H), Wm (b) = {Q(b, ωξ ) : ξ ∈ H, ξ = 1}. We recall that each φ ∈ P(A) gives rise, via the GNS construction, to an irreducible representation πφ : A → B(Hφ ) and there is a unit vector ξφ ∈ Hφ with φ(x) = ωξφ (πφ (x))). Conversely, given π ∈ Aˆ and a unit vector ξ ∈ Hπ we can get φ ∈ P(A) from φ(x) = ωξ (π(x)). ˆ For φ, ψ ∈ P(A) we write φ ∼ ψ if [πφ ] = [πψ ] in A. Theorem 7.1 (Formula A-1 [32]). For A a unital C ∗ -algebra, T ∈ E(A) given by (1.1), a = [a1 , a2 , . . . , a ] (a row) and b = [b1 , b2 , . . . , b ]t (a column), we have T = sup{tgm(Q(a∗ , φ1 ), Q(b, φ2 )) : φ1 , φ2 ∈ P(A), φ1 ∼ φ2 }. To state a version of Formula k that applies to general A, we need a generalisation of (2.1) (see [1, Theorem 5.3.12]); T k = sup T π k

(k ≥ 1), and T cb = sup T π cb .

ˆ π∈A

(7.1)

ˆ π∈A

We also need some more notation. For k ≥ 1, a (type I) factorial state of degree (at most) k on A is φ ∈ S(A) that is expressible as a convex combination φ=

k

ti φi

i=1

k with ti ≥ 0 (all i), i=1 ti = 1, φi ∈ P(A) (all i) and φi ∼ φj (1 ≤ i, j ≤ k). We write Fk (A) for these states. For each φ ∈ Fk (A) as above, there is an associated element of Aˆ given by the class [πφ1 ] of the GNS representation for φ1 . We denote this associated element of Aˆ by β(φ). Theorem 7.2 (Formula A-k [32]). For k ≥ 1 and T , a, b as in Formula A-1, we have T k = sup{tgm(Q(a∗ , φ1 ), Q(b, φ2 )) : φ1 , φ2 ∈ Fk (A), β(φ1 ) = β(φ2 )}.

142

R.M. Timoney

In short, Formula A-1 expresses T in terms of pairs or pure states which are related, and so a variation on numerical range ideas. To pass to Formula A-k, we take convex combinations of (at most) k pure states, but all the states must ‘belong’ to the same irreducible representation. 4∞ By using a k ≥ , or taking states in Ff (A) = k=1 Fk (A), we get: Theorem 7.3 (Formula A-CB [32]). For T , a, b as in Formula A-1, we have T cb = sup{tgm(Q(a∗ , φ1 ), Q(b, φ2 )) : φ1 , φ2 ∈ Ff (A), β(φ1 ) = β(φ2 )}. Moreover T cb = T and if supπ∈Aˆ dim Hπ := n < ∞ (that is if A is subhomogeneous), then T cb = T k for k = min(n, ).

8. Applications One question we could consider is this one: Question 8.1. Does Haagerup’s theorem T cb = uh hold for general C ∗ -algebras A, or for many C ∗ -algebras? Here we are taking u = j=1 aj ⊗ bj ∈ A ⊗ A and T ∈ E(A) as in (1.1). One way to make sense of uh is to consider A as a (unital) C ∗ -subalgebra of B(H) for some H and then use the earlier definition of uh for u ∈ B(H) ⊗ B(H). This is essentially relying on the injectivity of · h . The ideas outlined in §4 apply to T ∈ E(A) and show that we always have the inequality T cb ≤ uh . Vector states of B(H) restrict to give states of A, but there is no reason to expect the restrictions to be in P(A) or Ff (A). However, S(A) is the weak*-closed convex hull of P(A) and so we can use Formula-CB (see [7, Lemma 2.1]) to show that uh = sup{tgm(Q(a∗ , φ1 ), Q(b, φ2 )) : φ1 , φ2 ∈ co(P(A))}. (8.1) Comparing with Formula A-CB, we can see that strict inequality seems likely in T cb ≤ uh, at least unless we can resolve the discrepancy between Ff (A) on the one hand and co(P(A)) on the other. In Ff (A) we have rather restricted scope to include convex combinations of pure states. We do not actually need equality in the sets of states considered, but rather approximate equality of the relevant values (on aj a∗i and b∗j bi ). Essentially, if we are to have T cb = uh for all T ∈ E(A) we seem to need Ff (A) to be weak*-dense in S(A), which is a rather rare property. The class of C ∗ -algebras where this property holds is known to coincide with the prime C ∗ -algebras. (See [2] and [8, Proposition 2.2].) Primeness of A is equivalent to La Rb = 0 (on A) for nonzero a, b ∈ A, but is usually stated as the property that two nonzero ideals of A have nonzero product. It is a result of Mathieu (see [1, Proposition 5.4.11]) that T cb = uh holds for all u ∈ A ⊗ A if and only if A is a prime C ∗ -algebra. As yet, we cannot deduce this result of Mathieu from our remarks above because of the need to deal with pairs of states simultaneously in Formula A-CB.


143

However, this aspect turns out to be a technical issue, as the lemma below will imply. In work of Archbold and Batty [3], the states φ ∈ S(A) that are in the weak*closure of Ff (A) were characterised as those where ker πφ is a primal ideal of A (πφ being the associated GNS representation). An ideal Q ⊂ A is called n-primal (n ≥ 2) if whenever J1 , J2 , . . . , Jn are ideals of A with product J1 J2 . . . Jn = {0}, then there is some 1 ≤ j ≤ n with Jj ⊂ Q. We call Q a primal ideal if it is n-primal for each n, and write Primal(A) for the set of primal ideals of A. Included among ˆ By primal ideals are all primitive ideals (those of the form ker π with π ∈ A). a simple Zorn’s lemma argument, every primal ideal contains a minimal primal ideal. We write Min-Primal(A) for the set of minimal primal ideals of A, Prim(A) for the primitive ideals. Lemma 8.2 ([7, Lemma 2.2]). Let Q ⊂ A be a primal ideal and ψ1 , ψ2 ∈ S(A/Q) (equivalently φ1 , φ2 ∈ S(A) vanishing on Q). Then there exist commonly indexed nets (φ1,α )α and (φ2,α )α in Ff (A) so that β(φ1,α ) = β(φ2,α ) (all α), φ1,α → ψ1 and φ2,α → ψ2 (weak*-convergence). For u = j=1 aj ⊗ bj ∈ A ⊗ A, and I ⊂ A an ideal, we write uI = j=1 (aj + I) ⊗ (bj + I) ∈ (A/I) ⊗ (A/I). It is easy to show that uh ≥ uI h for any ideal I, and more generally that if I ⊂ J ⊂ A are ideals then uI h ≥ uJ h . From our Formula A-CB, the Lemma and (8.1) (applied to uQ ) we can immediately deduce a result of Somerset. Theorem 8.3 (Somerset [29]). Let A be a (unital) C ∗ -algebra, u = j=1 aj ⊗ bj ∈ A ⊗ A and T ∈ E(A) as in (1.1). Then T cb = sup{uQ h : Q ∈ Min-Primal(A)}. Corollary 8.4 (Mathieu). Let A be a (unital) C ∗ -algebra. Then A has the property that T cb = uh for each u = j=1 aj ⊗ bj ∈ A ⊗ A and T ∈ E(A) as in (1.1) if and only if A is prime. This corollary is evident from the theorem as the prime C ∗ -algebras are exactly those where Q = {0} is a (minimal) primal ideal, and for the converse consider the definition given above for primeness. (It was established by Weaver [34] that there are [necessarily nonseparable] prime C ∗ -algebras which are not primitive. See also [10].) To appreciate the result of Somerset, note that it is immediate from (7.1) and Haagerup’s theorem that T cb = sup{uP h : P ∈ Prim(A)}. Here is an example to show how, in general, Min-Primal(A) can go further down the ideal structure of A than Prim(A) does. Example. Let A be the C ∗ -subalgebra of C([0, 1], M3 ), the continuous functions on the unit interval with values in M3 , given by restrictions on x(0) and x(1/n)

144

R.M. Timoney

(n ≥ 1) as follows: ⎡

λ1 (x) x(0) = ⎣ 0 0

0 λ2 (x) 0

⎤ 0 0 ⎦, λ3 (x)

x

1 μ (x) = n 0 n

0 . θn (x)

Here λj (x) ∈ C (1 ≤ j ≤ 3), μn (x) ∈ M2 , θn (x) ∈ C. Then the minimal primal ideals of A are given by Qt = {x ∈ A : x(t) = 0} for t ∈ [0, 1]. For t = 0 or t = 1/n, Qt ∈ / Prim(A). In view of the result in Corollary 8.4, we may realise that Question 8.1 was a somewhat naive question. In particular, prime C ∗ -algebras A have trivial center Z(A) and we could observe that for a, b ∈ A, z ∈ Z(A) the elementary operator T corresponding to u = (az) ⊗ b − a ⊗ (zb) is T = 0. This leads us to consider the quotient of A ⊗h A by the closed linear span JA of {(az) ⊗ b − a ⊗ (zb) : a, b ∈ A, z ∈ Z(A)}. We use the notation A ⊗Z,h A for the quotient (A ⊗h A)/JA (with the quotient norm denoted · Z,h ). We call A ⊗Z,h A the central Haagerup tensor product (see [1, §5.4]). Question 8.5. For which C ∗ -algebras A do we have T cb = uZ,h for all u = j=1 aj ⊗ bj ∈ A ⊗ A and T ∈ E(A) as in (1.1). This question has a satisfying answer in terms of the Glimm ideals of A, which are the ideals I of A such that I ∩ Z(A) is a maximal ideal in Z(A) and generates I as an ideal of A. Somerset [29] showed that uZ,h = sup uG h , where the supremum is over all Glimm ideals G of A. It is known that every Q ∈ Min-Primal(A) contains a unique Glimm ideal, and then from Theorem 8.3 it is clear that a sufficient condition (for a positive answer) is that Min-Primal(A) coincides with the set of Glimm ideals of A. In [5], this was shown to be necessary as well. We quote now a result from [5, Theorem 17], which can be established using similar ideas but requires more detailed considerations. By θZ we mean the map from A ⊗Z,h A to CB(A) (the completely bounded linear operators on A with the CB norm) induced by θ : A⊗A → E(A) via continuous extension to the completion A ⊗h A and then passing to the quotient A ⊗Z,h A = (A ⊗h A)/JA . Theorem 8.6. Let A be a unital C ∗ -algebra. Fix ≥ 1. Then

holds for each u = (2 + 1)-primal.

j=1

θZ (u)cb = uZ,h aj ⊗ bj ∈ A ⊗ A if and only if every Glimm ideal in A is

Our aim was to show some examples where Formula A-CB suggest results that are not really obvious. There are other examples in [32, 7, 6] where considerations with the various formulae from §7 lead to new results that turn out to be best possible in various ways. We indicate some of these in the next section.


145

9. Generalisations for non-unital A Most of the results have versions in the context of C ∗ -algebras that are not necessarily unital, but we have simplified our exposition by restricting to unital A above. It is natural to consider the multiplier algebra M (A) of A in the case when A has no identity. We recall that the multiplier algebra is a universal object associated with A, the largest algebra that contains A as an essential ideal. It is normally accepted that to define E(A) in the non-unital case we should take those operators T on A expressible in the form (1.1) where ≥ 1 and aj , bj ∈ M(A). There is then a map Θ : M (A) ⊗ M (A) → E(A) given by Θ j=1 aj ⊗ bj = T and an associated inequality Θ(u)cb ≤ uh . We should then try to recast our results from the unital case to this more general setting, a setting that has a number of advantages. One obvious advantage is that it does allow left and right multiplication operators La and Ra , and inner derivations δa = Ra − La , (for a ∈ A or a ∈ M (A)) to be considered as genuine elementary operators. A rather basic fact is that if I ⊂ A is an ideal in the C ∗ -algebra A and T ∈ E(A), then T (I) ⊂ I. [See [11, Proposition 1.8.5] for the fact that an ideal of an ideal of a C ∗ -algebra is an ideal.] The multiplier algebra M (A) can be characterised (see [1, Chapter 1], for instance) by abstracting the properties of the pair (Lm |A , Rm |A ) of operators on A (for m an element of a C ∗ -algebra containing A as an essential ideal). It follows easily that for an ideal I ⊂ A and a, b ∈ M (A), the restriction to I of the map x → La Rb x is in E(I). More generally, T ∈ E(A) implies that the restriction T |I ∈ E(I). A fact which may be considered as reducing calculations to the unital case is that for T = Θ(u) ∈ E(A), u ∈ M (A) ⊗ M (A), we may also consider T = θ(u) ∈ E(M (A)) and we have Θ(u)k = θ(u)k (for each k ≥ 1) by an application of Kaplansky’s density theorem (see [5, Corollary 9]). This enables the formulae stated previously for unital A to be restated for general A at the expense of substituting M (A) for A in many cases. For instance [5, Corollary 9] provides an answer to the analogue of Question 8.5, as follows. Proposition 9.1. Let A be a C ∗ -algebra. Then the equality Θ(u)cb = uZ,h holds for all u ∈ M (A) ⊗ M (A) (where Θ(u) ∈ E(A) and · Z,h is the norm of M (A) ⊗Z,h M (A)) if and only if all Glimm ideals of M (A) are primal. A drawback of this statement, and of other statements we could deduce similarly, is that M (A) can be quite a lot more complicated than A itself. In [5, Example 12], there is an example where A is quite well behaved in the sense that its primitive ideal space Prim(A) is compact Hausdorff, while M (A) has Glimm ideals that are not 2-primal. One way to view Glimm ideals and primal ideals (a way that works for both unital and non-unital A) is that they are related to the pathological nature of the topology of Prim(A). A definition equivalent to the one given earlier is that an ideal

146

R.M. Timoney

I of A is n-primal when P1 , P2 , . . . , Pn ∈ Prim(A/I) (which we identify with ideals in Prim(A) containing I) implies there exists a net (Pα )α in Prim(A) converging to all of P1 , P2 , . . . , Pn (see [3]). It follows that Min-Primal(A) and Prim(A) are different if and only if Prim(A) is not Hausdorff. A Glimm ideal is the kernel of an equivalence class in Prim(A), where P, P ∈ Prim(A) are defined to be equivalent if f (P ) = f (P ) for each bounded continuous function f : Prim(A) → R. Thus, Prim(A) must be completely regular and Hausdorff for continuous functions on it to separate its points (and then Glimm(A) coincides with Prim(A)). What we learn from [5, Example 12] is that the pathology present in M (A), or in Prim(M (A)), may not be evident in Prim(A). Since results like Proposition 9.1 are necessary and sufficient conditions, we seem to have no way to state them in terms of conditions on A itself (without invoking M (A)). It is worth pointing out that the formulae from §7 do in fact carry over to the non-unital case without change. The multiplier algebra enters in via the considerations that lead to (8.1), where we must substitute P(M (A)) for P(A) when we take u ∈ M (A) ⊗ M (A). We can do better in some sense if we stick to the original kind of elementary operator (1.1) with the restriction aj , bj ∈ A, even when A is not unital, thereby forsaking the advantages of allowing aj and bj to be in M (A). This is the approach used in [7]. One hurdle is the selection of an appropriate definition of the central Haagerup tensor product A ⊗Z,h A for A not unital. Here we do still invoke the multiplier algebra, or more precisely its centre Z(M (A)), and consider the closed linear span JA in A ⊗h A of {(az) ⊗ b − a ⊗ (zb) : a, b ∈ A, z ∈ Z(M (A))}. Note however that Z(M (A)) is directly related to Prim(A) by the Dauns-Hoffman theorem. Then defining A ⊗Z,h A = (A ⊗h A)/JA (a deviation from the notation of [1, Definition 5.4.14]) we do get a map θZ : A⊗Z,h A → CB(A) induced naturally from θ, a direct generalisation of Theorem 8.3 (to allow non-unital A) is given by [7, Theorem 3.5], and [7, Theorem 3.9] generalises Theorem 8.6 above (to allow non-unital A). We quote [7, Theorem 3.9] next. Theorem 9.2. Let A be a C ∗ -algebra and θZ : A ⊗Z,h A → CB(A) the linear contraction given by θZ (a ⊗ b + JA )(x) = axb. Then (i) θZ is injective if and only if each G ∈ Glimm(A) is a 2-primal ideal of A. (ii) θZ is an isometry if and only if each G ∈ Glimm(A) is a primal ideal of A. The main goal in [7] was to investigate conditions on A less general than 2-primality of all Glimm ideals, but more general than primality of all Glimm ideals, under which we can conclude that θZ is an isomorphism onto its range. −1 This means conditions under which L(A) := θZ < ∞ (assuming, as we will do below, that θZ is injective). The results are more satisfactory in the case that A is stable, or by considering the map θZ not just on A but on all matrix algebras Mn (A) at once.


147

Yet another way to characterise n-primality of an ideal I of nA is to say that whenever P1 , P2 , . . . , Pn ∈ Prim(A/I), then their intersection j=1 Pj is primal. If we exclude repetitions among the Pj and also exclude the possibility of containments Pi ⊆ Pj , we can restate the condition. We say that an m-tuple P1 , P2 , . . . , Pm ∈ Prim(A) is ‘admissible’ if there is no i = j with Pi ⊆ Pj . Then n-primality of I is equivalent to m j=1 Pj being primal for all admissible m-tuples P1 , P2 , . . . , Pm ∈ Prim(A/I), for all m ≤ n. In [7] we consider Glimm ideals G of A and for each admissible N -tuple P1 , P2 , . . . , PN ∈ Prim(A/G) we look at the associated collection of subsets E ⊂ {1, 2, . . . , N } such that j∈E Pj is primal. These collections are abstract simplicial complexes (meaning collections of subsets containing all singleton subsets of the vertex set {1, 2, . . . , N } and closed under taking subsets; these are determined by their maximal members), and we say the simplicial complexes that arise in this way (for some G ∈ Glimm(A)) are ‘linked to the ideal structure of A’. When A is stable we show in [7, §6] that the isomorphism constant L(A) depends only on the simplicial complexes linked to the ideal structure of A, hence only on the topological space Prim(A). There are just a few examples given in [7, §7] where we can compute L(A), but these are sufficient to show that L(A) can be infinite, any positive integer, or satisfy 1 < L(A) < 1 + ε (for any prescribed ε > 0). It remains open whether there are restrictions on the possible values of L(A). We refer to [7] for full details on these matters. Finally we mention that there are other applications of the formulae from §7 to be found in [6], where the notion of the upper multiplicity of an irreducible representation of a C ∗ -algebra is linked to considerations of norms of elementary operators.

10. Postscript There have been some recent works (since the workshop) which are sufficiently pertinent to our overview above of earlier work as to deserve mention here. I. Gogić [16, 15] has developed the work of B. Magajna [24, 23] cited in §2 above, which deals with understanding the closure in the CB norm (or in operator norm) of E(A) (in the case of [23] the closure in the topology of pointwise norm convergence of {T ∈ E(A) : T cb ≤ 1}). Magajna considers the relationship of these closures with ICB(A) = {T ∈ CB(A) : T (I) ⊆ I for each ideal I ⊆ A}. In [24], he characterized those separable A such that Θ(M (A) ⊗ M (A)) is dense in ICB(A) in CB norm (as those which are a finite direct sum of homogeneous C ∗ algebras of finite type). On the other hand Gogić [16] showed that if A is separable and has ΘZ (M (A) ⊗Z,h M (A)) = Θ(M (A) ⊗ M (A)) then A must be subhomogeneous and furthermore have a finite type property. Amongst other results, [15, §6] exhibits an example of a separable 2-subhomogeneous unital A such that E(A) is complete in the operator norm, so that the range of ΘZ = θZ is just E(A), moreover E(A) ICB(A), and there is a derivation δ of A that is in E(A) but is

148

R.M. Timoney

not inner (i.e., δ = δc for c ∈ A). The main thrust in [15] is the relation between the range of θZ and derivations on A. In private correspondence, I. Gogić has established the following result. Proposition 10.1 (Gogić). Assume A is an m-subhomogeneous C ∗ -algebra such that each Glimm ideal of A is 2-primal. Then L(A) < ∞ if and only if sup{| Prim(A/G)| : G ∈ Glimm(A)} = N < ∞ (for | Prim(A/G)| the cardinality of Prim(A/G)). The justification is that Aˆ can be identified with Prim(A) (since A is liminal) and [13, Corollary 1 on p. 388] then implies that | Prim(A/Q)| ≤ m for each Q ∈ Primal(A). Hence [7, Proposition 7.15] implies L(A) ≥ N/m, while [7, Corollary 7.11] (Proposition 9.1 for N = 1) gives L(A) ≤ max(1, N − 1). Using constructions inspired by generalising the simplicial spoke algebras of [7, 4.6] to infinitely many ‘spokes’ (or elaborating [15, §6]) together with Proposition 10.1, one can construct (unital separable) subhomogeneous C ∗ -algebras with all Glimm ideals 2-primal but L(A) = ∞ (and hence E(A) not closed in CB norm, or in operator norm). Further properties considered in [15] can also be satisfied by such examples. (It is also possible to find subhomogeneous A to show that the hypothesis in Proposition 10.1 on 2-primality cannot be removed.) A separate development is recent work of Archbold and Somerset [4] which sheds light on the relation between the primality properties of the Glimm ideals of M (A) and properties of A itself. A striking result of [4] applies to separable C ∗ -algebras A and states that all Glimm ideals of M (A) are primal (respectively n-primal for some n ≥ 2) if and only if all Glimm ideals of A are both primal (respectively n-primal) and ‘locally modular’. The definition of local modularity of G ∈ Glimm(A) is subtle. Let YG denote the boundary and UG the interior of Prim(A/G) considered as a subset of Prim(A). G is called locally modular if for each P ∈ YG there isan open neighbourhood V of P ∈ Prim(A) such that A/I is unital where I = {R : R ∈ V \ UG }. There are several other significant results in [4] relating the Glimm ideals of A with ideals of M (A). Amongst the key ingredients used in [4] are recent results of A. Lazar [19, 20], which are expressed in topological terms but apply in particular to locally compact (not necessarily Hausdorff spaces) such as Prim(A). According to the formulation of primality in §9 (with nets), the topological interpretation of primality concerns (closed) limit sets. The considerations in [20] concern topologies on the set of closed limit sets of a locally compact space X and how they relate to the complete regularisation of X. An important result from [20] implies that two natural topologies on Glimm(A) (the cr topology being the weak topology induced by scalar-valued continuous functions on Prim(A), and the quotient topology from Prim(A)) must coincide when A is σ-unital (in particular if A is separable). Delicate topological considerations are also a feature of the techniques used in [4].


149

Acknowledgment The author would like to thank Martin Mathieu for constructive comments on an earlier draft of this paper.

References [1] P. Ara and M. Mathieu, Local multipliers of C ∗ -algebras, Springer Monographs in Mathematics, Springer-Verlag London Ltd., London (2003). [2] R.J. Archbold, On factorial states of operator algebras, J. Funct. Anal. 55 (1984), 25–38. [3] R.J. Archbold and C.J.K. Batty, On factorial states of operator algebras. III, J. Operator Theory 15 (1986), 53–81. [4] R.J. Archbold and D.W.B. Somerset, Separation properties in the primitive ideal space of a multiplier algebra, in preparation. [5] R.J. Archbold, D.W.B. Somerset and R.M. Timoney, On the central Haagerup tensor product and completely bounded mappings of a C ∗ -algebra, J. Funct. Anal. 226 (2005), 406–428. [6] R.J. Archbold, D.W.B. Somerset and R.M. Timoney, Factorial states, upper multiplicity and norms of elementary operators, J. Lond. Math. Soc. (2) 78 (2008), 707–722. [7] R.J. Archbold, D.W.B. Somerset and R.M. Timoney, Completely bounded mappings and simplicial complex structure in the primitive ideal space of a C ∗ -algebra, Trans. Amer. Math. Soc. 361 (2009), 1397–1427. [8] C.J.K. Batty and R.J. Archbold, On factorial states of operator algebras. II, J. Operator Theory 13 (1985), 131–142. [9] R. Bhatia and P. Rosenthal, How and why to solve the operator equation AX −XB = Y , Bull. London Math. Soc. 29 (1997), 1–21. [10] M.J. Crabb, A new prime C ∗ -algebra that is not primitive, J. Funct. Anal. 236 (2006), 630–633. [11] J. Dixmier, Les C ∗ -algèbres et leurs représentations, Cahiers Scientifiques, Fasc. ´ XXIX, Gauthier-Villars & Cie, Editeur-Imprimeur, Paris (1964). [12] E.G. Effros and Z.-J. Ruan, Operator spaces, London Mathematical Society Monographs. New Series, vol. 23, Oxford University Press, New York (2000). [13] J.M.G. Fell, The dual spaces of C ∗ -algebras, Trans. Amer. Math. Soc. 94 (1960), 365–403. [14] C.K. Fong and A.R. Sourour, On the operator identity Ak XBk ≡ 0, Canad. J. Math. 31 (1979), 845–857. [15] I. Gogić, Derivations which are inner as completely bounded maps, Operators and Matrices 4 (2010), 193–211. [16] I. Gogić, Elementary operators and subhomogeneous C ∗ -algebras, Proc. Edinb. Math. Soc. (2) (to appear). [17] U. Haagerup, The α-tensor product of C*-algebras, unpublished manuscript, Univ. of Odense (1980).

150

R.M. Timoney

[18] P.R. Halmos, A Hilbert space problem book, Graduate Texts in Mathematics, vol. 19, Springer-Verlag, New York, second ed. (1982), encyclopedia of Mathematics and its Applications, 17. [19] A.J. Lazar, Hyperspaces of closed limit sets, Methods Funct. Anal. Topology 16 (2010), 158–166. [20] A.J. Lazar, Quotient spaces determined by algebras of continuous functions, preprint arXiv:0811.3280. [21] G. Lumer and M. Rosenblum, Linear operator equations, Proc. Amer. Math. Soc. 10 (1959), 32–41. [22] B. Magajna, A transitivity theorem for algebras of elementary operators, Proc. Amer. Math. Soc. 118 (1993), 119–127. [23] B. Magajna, Pointwise approximation by elementary complete contractions, Proc. Amer. Math. Soc. 137 (2009), 2375–2385. [24] B. Magajna,Uniform approximation by elementary operators, Proc. Edinb. Math. Soc. (2) 52 (2009), 731–749. [25] M. Mathieu (ed.), Elementary operators and applications, World Scientific Publishing Co. Inc., River Edge, NJ (1992), in memory of Domingo A. Herrero. [26] M. Mathieu, The norm problem for elementary operators, in Recent progress in functional analysis (Valencia, 2000), North-Holland Math. Stud., vol. 189, NorthHolland, Amsterdam (2001), pp. 363–368. [27] M.A. Nielsen and I.L. Chuang, Quantum computation and quantum information, Cambridge University Press, Cambridge (2000). [28] E. Saksman and H.-O. Tylli, Multiplications and elementary operators in the Banach space setting, in Methods in Banach space theory, London Math. Soc. Lecture Note Ser., vol. 337, Cambridge Univ. Press, Cambridge (2006), pp. 253–292. [29] D.W.B. Somerset, The central Haagerup tensor product of a C ∗ -algebra, J. Operator Theory 39 (1998), 113–121. [30] J.G. Stampfli, The norm of a derivation, Pacific J. Math. 33 (1970), 737–747. [31] R.M. Timoney, Computing the norms of elementary operators, Illinois J. Math. 47 (2003), 1207–1226. [32] R.M. Timoney, Some formulae for norms of elementary operators, J. Operator Theory 57 (2007), 121–145. [33] J. Watrous, Distinguishing quantum operations having few Kraus operators, Quantum Inf. Comput. 8 (2008), 819–833. [34] N. Weaver, A prime C ∗ -algebra that is not primitive, J. Funct. Anal. 203 (2003), 356–361. Richard M. Timoney School of Mathematics Trinity College Dublin 2, Ireland e-mail: [email protected]

Open Problems This section contains problems that were either stated during the Problem Session at the workshop or arose in connection with the contributions to these proceedings. At the time of going into print, they appear to be open. We start with some common notation. For a Banach algebra A, let E(A) be the algebra of elementary operators on A. If a, b ∈ A, let Ma,b ∈ E(A) be the operator given by Ma,b (x) = axb. Also let La , Rb be the elementary operators on A given by La (x) = ax, Rb (x) = xb. Therefore, a general elementary operator on A is of the form Ra,b = nj=1 Maj ,bj , where a = (a1 , . . . , an ) and b = (b1 , . . . , bn). For a Banach space X , let B(X ) (resp. K(X )) denote the algebra of bounded linear (resp. compact) operators on X . If H is a Hilbert space H, let C2 (H) be the ideal of all Hilbert–Schmidt operators on H. The problems will be listed alphabetically by author. L. Fialkow Do there exist elementary operators other than derivations such that Ra,b|S = Ra,b for every symmetrically normed ideal S in B(H)? R. Harte (i) Relate the conditions on an elementary operator of being hyponormal, strong Fuglede and *-hyponormal. (ii) Let A be a Banach algebra and a, b ∈ A. The operator D = La − Rb satisfies the algebraic condition D(xy) = xD(y) − D(x)y. Similarly, one can write down an algebraic expression for D(xyz). Are there similar algebraic conditions for the operator Ma,b ? (iii) Anderson and Foias ([AF, Theorem 5.8]) show that, if 0 = p∗ = p = p2 = 1 ∈ A = B(H) for Hilbert space H, then p ∈ A, and hence Lp , Rp ∈ B(A), are Hermitian while the product Lp Rp is not. Is it the case that Lp Rp is in the “Palmer subspace” Reim B(A) = Re B(A) + iRe B(A)? What exactly is the numerical range of Lp Rp ? (iv) Alternatively is there in a Banach algebra implication ab = ba = h + ik with hermitian a, b, h, k implies ab = h is hermitian? [AF] J. Anderson and C. Foias, Properties which normal operators share with normal derivations and related properties, Pacific J. Math. 61 (1975) 133–325.


151

152

Open problems

B. Magajna ∞ ∗ (i) Let φ be a mapping on B(H) of the form φ(x) = n=1 an xan , where ∞ ∗ a a = 1 and the sums are convergent in the strong operator topoln=1 n n ogy. Suppose that H is separable and that φ preserves the ideal of compact operators. (For example, let an be orthogonal projections with sum 1.) Can φ be approximated pointwise in norm by a net of completely contractive elementary operators? (ii) Suppose that the normal and the singular part of a complete contraction φ on a von Neumann algebra can both be approximated pointwise in norm by nets of completely contractive elementary operators. Does then φ necessarily admit such an approximation? The answer is affirmative for completely positive maps by Corollary 4.2 in my paper in this volume. But the case of completely contractive maps can not be easily reduced to completely positive maps, perhaps not even on injective von Neumann algebras, where one could try to use the well-known 2×2 matrix technique. Namely, maps that admit pointwise approximation by elementary operators must preserve closed ideals, while the Arveson extension theorem for completely positive maps on operator systems does not automatically provide extensions with this property. M. Mathieu (i) Let A and B be Banach algebras or C*-algebras. Suppose that the algebras E(A) and E(B) are isomorphic. Does it follow that A and B are isomorphic? (ii) Following on from our article in this volume, characterise when a length two elementary operator Ma,b + Mc,d is a spectral isometry. P. Rosenthal Let A be either B(H), K(H) or C2 (H), for some Hilbert space H. (i) Characterise the equivalence classes of elementary operators arising from the relation of unitary equivalence or similarity. (ii) Find all invariant subspaces of particular elementary operators. (iii) Determine when a given elementary operator is subnormal or quasi-normal. V.S. Shulman and Yu.V. Turovskii (i) Let a, b, ai , bi ∈ K(X ), i = 1, . . . , n. Does the elementary operator La + Rb + n i=1 Mai ,bi possess a “non-trivial invariant” subspace? Here “non-trivial” means different from the closed ideals of B(X ). For a more definite formulation one can consider the restriction of R to the smallest closed ideal A(X ), the closure of the ideal of finite rank operators, and ask about invariant subspaces nontrivial in the usual sense, that is, different from 0 and A(X ). The problem seems to be interesting in itself as any concrete version of the invariant subspace problem. But a fresh idea leading to its solution could have applications, for example to the conjecture of W. Wojty´ nski [4] that a Lie algebra of Volterra operators cannot be topologically simple (see also [1]).

Open problems

153

It should be added that for operators R1 (x) = ax+xb and R2 (x) = ni=1 ai xbi the existence of invariant subspaces can be easily verified: R2 is compact, R1 commutes with the compact operator Ma,b (thus its non-transitivity follows from [2]). This gives a hope that the problem is not wild. (ii) A stronger version of the previous problem is: Let R = ni=1 Mai ,bi be an elementary operator on A(X ), and for each i at least one of the operators ai , bi is compact. Does R have a non-trivial invariant subspace? (iii) Let us denote, for any Banach algebra A, by Mul(A) the norm closure in B(A) of the algebra E(A) of all elementary operators. Let A be a compact radical Banach algebra, for example a norm-closed algebra of Volterra (= compact quasinilpotent) operators. Is the algebra Mul(A) radical? It should be said that the Jacobson radical of Mul(A) contains all operators n i=1 Lai Rbi . Hence the semisimple quotient Mul(A)/Rad(Mul(A)) is generated by two subalgebras M1 and M2 (the images of the algebras of left and right multiplications) which consist of quasinilpotent elements and satisfy the condition M1 M2 = M2 M1 = 0. Is this possible? Partial positive results can be found in Section 5 of [3]. [1] M. Breˇsar, V.S. Shulman, Yu.V. Turovskii, On tractability and ideal problem in non-associative operator algebras, (to appear in Integral Equations and Operator Theory). [2] V.I. Lomonosov, Invariant subspaces for operators commuting with compact operators, Funct. Anal. Appl. 7 (1973), 213–214. [3] V.S. Shulman, Yu.V. Turovskii, Tensor radical and its applications to spectral theory of multiplication operators, (this volume). [4] W. Wojty´ nski, Engel’s theorem for nilpotent Lie algebras of Hilbert-Schmidt operators, Bull. Acad. Polon. Sci. 24 (1976), 797–801.

E. Saksman and H.-O. Tylli Let X be a Banach space. Suppose that an elementary operator R ∈ E(B(X )) is invertible. How far is the inverse R−1 from being an elementary operator? In ·

particular, is it true that R−1 ∈ E(B(X )) ? Note that, if the operator R from the last question has the form R = La − Rb , ·

for some a, b ∈ B(X ), then R−1 does belong to E(B(X )) , see [R]. For X = 2 one may deduce from a result of Magajna [M] that R−1 belongs at least to the strong operator topology closed hull of E(B(2 )) for any invertible R ∈ E(B(2 )). [M] B. Magajna: A transitivity theorem for algebras of elementary operators, Proc. Amer. Math. Soc. 118 (1993), 119–127. [R] M. Rosenblum: On the operator equation BX − XA = Q, Duke Math. J. 23 (1956), 263–269.

Elementary operators and their applications

Hankel Operators and their Applications

Linear Operators and their Spectra

Linear Operators and their Spectra

Elementary Topology and Applications

Elementary Particles and Their Interactions

Elementary topology and applications

Elementary Topology and Applications

Elementary topology and applications

Positive operators in Banach spaces and their applications

Positive operators in Banach spaces and their applications

Hankel Operators and Their Applications (Springer Monographs in Mathematics)

An elementary treatise on differential equations and their applications

Exponentially dichotomous operators and applications

Sturm-Liouville Operators and Applications

Nonlinear Integral Operators and Applications

Nonlinear integral operators and applications

Orders and their Applications

Metagraphs and Their Applications

Wavelets and their applications

Metagraphs and Their Applications

Wavelets and their applications

Cellulases and Their Applications

Metaclasses and Their Applications

Geosynthetics and their applications

Metaclasses and Their Applications

Wavelets and their Applications

Wavelets and Their Applications

Semigroups and their applications

Elementary Probability for Applications

Baecklund transformations and their applications

Elementary operators and their applications

Hankel Operators and their Applications

Linear Operators and their Spectra

Linear Operators and their Spectra

Elementary Topology and Applications

Elementary Particles and Their Interactions

Elementary topology and applications

Elementary Topology and Applications

Elementary topology and applications

Positive operators in Banach spaces and their applications

Positive operators in Banach spaces and their applications

Hankel Operators and Their Applications (Springer Monographs in Mathematics)

An elementary treatise on differential equations and their applications

Exponentially dichotomous operators and applications

Sturm-Liouville Operators and Applications

Nonlinear Integral Operators and Applications

Nonlinear integral operators and applications

Orders and their Applications

Metagraphs and Their Applications

Wavelets and their applications

Metagraphs and Their Applications

Wavelets and their applications

Cellulases and Their Applications

Metaclasses and Their Applications

Geosynthetics and their applications

Metaclasses and Their Applications

Wavelets and their Applications

Wavelets and Their Applications

Semigroups and their applications

Elementary Probability for Applications

Baecklund transformations and their applications

Recommend Documents